Molly Fish – The Care, Feeding and Breeding of Mollies

Quick Stats

Minimum Tank Size: 10 Gallon (Short-finned Molly), 20 Gallon (Sailfin Molly)
Care Level: Easy
Water Conditions: 6.5-8 and Moderately Hard to Very Hard (Short-finned Molly), 7.5-8.2 and Hard to Very Hard (Sailfin Molly)
Temperature: 72-82 F (25-28 C)
Maximum Size: 4 inches (10 cm) Short-finned Molly, 6 inches (15 cm) Sailfin Molly

Mollies are one of those most popular fish in the aquarium hobby, and most people have had at one time, at least one of these gentle and attractive live bearers in their fish tank. Most people mistakenly believe that there is only one species of molly available in fish stores, but there are actually three separate species available in the aquarium trade. However, at this point, most of the mollies available are so hopelessly hybridized, that it would be impossible to determine which species they originated from.

If you ever come across wild mollies in a fish stores, they will usually come from one of two species. The first species is sailfin mollies (Poecilia latipinna) which are found in the coastal waters of the southeast United States and around the Gulf of Mexico. These are among the most common type of molly, and are incredibly prolific breeders in the wild. Because they live in coastal regions, they have a high tolerance for brackish water, and have even been found living in the ocean.

The other species that is commonly found in the aquarium trade is the short-finned molly (Poecilia sphenops). These fish are found in Mexico, Central America and in the northern parts of South America. The short-finned molly adapt most readily to the home aquarium, and is one of the easiest beginner fish available . Like their sailfin molly relatives, they can also tolerate high salinity in water, and do well in both brackish and freshwater tanks.

There is a third species of mollies that is occasionally found in the aquarium hobby, but it is far rarer than either the short-finned molly or the sailfin molly. It is the Mexican sailfin molly (P. velifera), and is very difficult to breed and keep in aquariums, though it does well in outdoor ponds. It is found in the wild in the coastal regions of Yucatan in Mexcio, and like the other two species, tolerates a very high level of salinity in the water.

Housing Molly Fish

The short-finned molly is undemanding when it comes to providing them with an aquarium, and most will happily live in a 10 gallon aquarium. But if you really want them to thrive, they should be provided with at least a 29 gallon long tank.

Sailfin mollies on the other hand, grow much larger than their short-finned cousins, and require a minimum tank size of at least 29 gallons. But like many other large fish in the hobby, the fish will do much better over the long run if they are provided with a large tank right off the bat. Larger tanks also have the benefit of offering a more stable environment for the fish, since they aren’t as prone to the sudden water quality fluctuations that smaller tanks suffer from.

All of the species of mollies that are commercially available are docile fish, and will do well in any peaceful community tank. And emphasis should be placed on the word peaceful – if there are any aggressive fish in the tank, the wide-bodied and passive mollies will be bullied mercilessly.

When it comes to filtration, mollies do much better when the tank is over-filtered. They aren’t necessarily a messy fish, but some of the hybrids are more susceptible to disease than some of the other beginner fish, and do better when living in well maintained water. The best filters to use are either a high quality hang-on-back filter, or a canister filter if you can afford the hefty price tag that comes with it (Click here for HOB filter reviews).

One question that comes up often with mollies, is “do mollies need salt in their water?” The answer to this question isn’t simple, and a lot depends on what species of molly you have. As with most fish, mollies do benefit with some aquarium salt in the water, but nothing has conclusively shown that mollies need to be kept in water with salt added. In fact, most mollies will thrive in a completely fresh water aquarium, with absolutely no salt added.

The myth that mollies have to have a large amount of salt in the aquarium likely stemmed from some of the hybrids being more sickly than other types of fish. Like any fish that has been over-bred (neons come to mind), the genetic line can sometimes be weakened, and the fish become more susceptible to diseases. Many people then falsely attribute the illness to a lack of salt, and a myth is born.

I have kept mollies for close to a decade now, and they have never been in a tank with any salt added. Aside from one minor disease outbreak a few years ago (from an improperly quarantined fish), they have always been healthy and have had no problems breeding in large numbers. In fact, many of the mollies that I currently keep, are descendants of the original mollies.

Feeding Molly Fish

In the wild, mollies are omnivores and feed on small invertebrates, algae and plant matter. In the home aquarium, they should be fed a high quality flake food, and a variety of vegetables. One of the best prepared foods that you can feed them is New Life Spectrum Small Fish Formula. Their favorite vegetables are blanched zucchini medallions, cucumber medallions and shelled peas. If you don’t have the time to cut and blanch vegetables, then a spirulina based pellet or flake can take the place of plant matter in their diet.

In order to get them into breeding condition, they should also be fed live or frozen foods as a treat. Their favorite frozen foods are bloodworms, daphnia, and brine shrimp. They will also greedily accept any live foods that you can find to feed to them, with live blackworms, bloodworms, daphnia and brine shrimp being special favorites.

Breeding Molly Fish

The molly fish is a live-bearer, which means that its eggs are internally gestated until highly developed fry are born. Because it’s doesn’t scatter its eggs, there is usually a high survival rate among its fry (as long as the tank doesn’t have any large predators).

Like most other live-bearers, breeding mollies is incredibly easy. It really boils down to fish + water = hordes of babies. But as with other easy to keep live-bearers like guppies, you have to be careful what ratio of sexes you keep in the aquarium.

In any aquarium that contains both male and female mollies, you should try to maintain a ration of three females to every male. Male mollies will relentlessly mate with any females in the tank, and if their attention isn’t divided between several females, then the female they focus on will eventually become highly stressed and may even die from the constant mating attempts.

If mollies are in a community tank, then the best way to ensure some of their fry survive is to add some floating plants or some dense plants like Java moss. While the babies are born highly developed, their small size will still allow for larger fish to eat them. The more plants that an aquarium has, the better chance that some fry will reach adulthood. Floating plants are the best choice, as fry can easily hide from adult fish in the dense vegetation near the surface.


  1. says

    Hi Seymour,
    I enjoyed your article, but I’m not having the success with mollies that you describe! I hope you can help me diagnose the problem…
    I’ve a 30 gallon freshwater tank, 82 degrees, regularly test and change water & filter…I’ve not added salt though.

    For some reason, I’ve lost 3 pairs of black mollies in the past 4 months since I started my aquarium. I can never see anything visibly wrong with them. I even bought the last pair at a different store, “just in case.”

    The 2 gold mollies are growing fine, the 2 gold fish and 5 neon tetras are thriving, but I’ve lost guppies also (1 male and about 5 females.) I have noticed the guppies get pale and listless a day before they die, but I don’t see any physical defects before then.

    It’s really distressing to see these pets die, so until I understand what’s wrong I’m not going to buy any more fish.
    Do you have any ideas?

    • Matthew Seymour says

      It sounds like your doing most things right already. The one thing that you might want to consider is lowering the temperature a bit. Quite a few of the fish in your aquarium prefer cooler water, and you may actually have low oxygen at that temperature, depending on the size of the goldfish in there.

      The mollies, guppies and the goldfish especially will appreciate a temperature that is a little lower, and if you can in the future, you might want to consider moving the goldfish to a tank that isn’t heated.

      If a lower temp doesn’t work, then you just might be getting unlucky with your fish. I’ve always found that the dalmation and black mollies are a little more sensitive and die for unexplained reasons.

      • says

        Thank you so much!
        - I’ll lower the temp to say – 78? I lost another guppy yesterday but I discovered the nitrites had spiked, so I did a 25% water change and will do another in 2 days. I’m going to feed a little less too.

        It’s comforting to learn the black mollies have seemed more sensitive and to die more often in your experience.

        The goldfish are the hardiest of all – I’ve never lost any! And they’re growing much faster than I expected, so it interested me that they may be using up oxygen at the current temperature. The comet is already 4″ nose to tail, the fantail 2.5.”

        Again – thank you for your help!

        • just CHASE says

          you are changing the water too much and a filter both have good bacteria in it that’s needed to lower the nitrates and break down waste. put in a lot of live plants get in that gravel with turkey baster or gravel pump clean up that waste and leave the filter going without changing it too oftenand not over feed the fish

          • susan says

            I did the same thing when I first inherited this 20 gallon fish tank…I changed once a week and over fed. But after using a test strip I change out the filter once a month now (but rinse it thoroughly after 2 weeks of use) and take out only 10% of the old water. I have been given so many different numbers from FISH store employees…10%, 20% or 25%…so confusing! Since my (tested) water source is good “well” water I don’t need to add a purifier when adding new water. I wish taking care of fish was as easy as caring for my dogs and cats…chuckle.

  2. says

    Great article. I am new to the fish scene as well. I prefer animals with fur. But one of the new black mollies is becoming very aggressive. He is bugging the heck out of the older molly as well and has chased his store tank buddy to the point of exhaustion. Since I haven’t been able to determine the sex of any of my fish (the store I purchased them from didn’t know either) could this aggressive behavior be a male’s mating drive? I have had this new batch of “teenage” fish for 2 months and he is the only one who is aggressive. The other fish are at least 2 years old and are barely tolerating his behavior. I have named this rascal Darth Vader. Any advice?

    • Matthew Seymour says

      In my experience, male mollies can be very aggressive to other males mollies and will spend the rest of their time trying to mate with any females in the tank. The best way to deal with this behavoir is to create different areas in the fish tank. This can be done by creating breaks in their line of sight, either through plants (real or fake) or with decorations.

      When you create areas where the molly can’t see the other mollies, it will usually calm down for a while. At the very least it will the other mollies a place to hide and recover a little bit.

      Beyond that, the only other option if it becomes a serious problem is to put in a tank divider. But that’s usually not a very good idea.

      • Susan says

        DARTH VADER LIVES in my 20 gallon tank!
        You are NOT going to believe this. Two weeks ago my male “tween” Dalmatian Molly kept antagonizing the female I purchased (with him on the same date). She found a place to hide in one of my castles…but eventually she lost her battle and died. Then an older female (about 2 years old) that I already had, turned the tables and began going after that little male.
        A couple of minutes later I looked over at the tank and noticed the little male was floating belly up in the tank. Since I thought he was dead I picked him up and placed him into the trash can. About 18 minutes later I pitched a cough drop wrapper into the same trash can…and he moved.OMG!!! I picked him up and tossed him back into the tank. YES…he was still alive. That’s when I gave him the name Darth Vader.
        I had no idea fish could survive outside of water for more than a couple of minutes until I went online and read more about Mollies and other fish. The article said some fish will breath through their skin until they are back in water. Well needless to say Darth Vader is alive and well but he has eased up on being as aggressive as before. I guess my old gal gave him a well deserved SMACK down that day.
        Since I purchased this last batch from PetCo…they kept their males and females in one tank…something I had never experienced before. But now I know the physical differences and will make sure not to buy any more males…they are too aggressive!
        I am still not sure who killed 2 of the 4 Platties and 1 of the 2 Guramies I bought along with Darth Vader and his sister that day…as the new fish were small. However I have 3 adult females (all Mollies) and one adult Algae Eater (about 4 inches long) that were already tenants before the new kids arrived.

        • Matthew Seymour says

          That is one tough fish. lol. I’ve had some catfish live out of water for a while, but I’ve never heard of a molly surviving that long.

          Is your algae eater a Chinese algae eater by any chance? Those are extremely aggressive fish and can wipe out tanks filled with more docile fish. They are generally good community fish when they are younger, but as they age they need to be kept with semi-aggressive fish only.

          • Susan says

            Hi…I honestly don’t know the difference in algae eaters…this one is medium brown with dark spots…looks like a mini shark…but this is my 3rd algae eater and they are always calm and quiet…it was abut 1 1/2 long when I bought it 6 months ago…it is now about 3 inches long…I hear there are “Pigmy” algae eaters??? I am kind of new to fish.
            Oh here’s any update on Darth Vader…my Guramie (sorry about the spelling) has been giving him the SMACK DOWN the last 2 days and now he is hiding out. Basically I am educating myself online about males and females. It appears DV is a male in a tank full of females…chuckle. I think they are fed up with his teenage antics! LOL! From now on I am going to buy fish from the tropical fish stores in town as they keep the males and females in separate tanks.
            I am still curious about the loss (eating) of the other new fish that I bought along with DV and his sister…?????

          • Susan says

            Maybe you can assist me. This is what I currently have in my 20 gallon tank.
            A 2 year old Dalmatian Molly, a 2 year old gold colored Molly, a 2 year old Algae eater, two 6 month old orange and black Platties (bought 4…2 were eaten), I bought 2 Guramies (one was chased by Darth Vader and eventually died from exhaustion) and 2 six month old Dalmatian Mollies (D Vader and his sister…she died from being chased by DV as well and died over about 2 weeks ago). My one remaining Guramie is thriving, becoming as large as the older Dalmatian and is now becoming aggressive with D Vader.
            The water is set at 76-78 on an auto temp thermostat. I change out the water every 2 weeks…only 10%…all levels read normal…and then I seem to see a change in behavior when ever I add a teaspoon of aquarium salt…so I have stopped the salt for awhile However I keep reading articles about salt and electrolytes…so I am confused…I do however keep the salt levels down. I change the charcoal filter once a month as well. i feel them flakes and granules…a little in the morning and again around 5:00 pm…I might be over feeding them. I thought if I fed them twice a day they would stop eating each other…uggg!
            I am a high school teacher and am doing as much research on these little guys as I can…but I keep finding information that differs.

  3. Deborah Rainey says

    dd one teaspoon of aquarium salt per 5 gallons and keep the ph at 7.5. They are doing well.

  4. marvella nicodemus says

    I unknowingly purchased a male and a female Molly … I had a male hippy and a Chinese algae eater already in the tank … These other fish just mysteriously disappeared one at a time. Shortly after the last disappeared there were babies born … I am just wondering if it is common for mollies to eat other fish in the tank. I thought they were docile and a good communal fish

    • Matthew Seymour says

      It’s likely the Chinese Algae eater. While they are docile while they are young, they get incredibly mean and aggressive as they get older. They will kill all sorts of fish and I would blame them in this case.

  5. susan says

    DARTH VADER is getting a SMACK DOWN….

    My new female Gourami (I bought her the same day I bought DV) is really after this guy…She is now twice the size of DV and chases him into a corner near the filter. I didn’t think Gouramis were aggressive…it’s strange as she has bonded with an older female Dalmatian Molly that Darth Vader had been chasing for a month. I can’t believe the action that happens within my tank and the personality each fish possesses.

  6. sally says

    HI, I have only had a tank for 6 months, 200litres, temp 24.5, I have tetras,danio’s, guppies, plecotimus,swordtails, cory’s and 2 black male sailfin mollies. All the fish have been doing really well, 10 wee babies already. Ph around 6.5 to 7. Heeps of really healthy plants. Sadly my male swordtail is a real mean fish, I have not seen him chase the mollies, but he chases any red/orange fish. My main concern is my sailfins, they started a week ago producing alot of white slime on them, rather like a fungus, but I was assured it was just a sign of stress, I have since fed less, used more stresszyme to help stablisie their scales/fins, but it just wont clear up. And now one of them has a swollen eye? I really dont want to use salt as i believe this is not good for tetra’s. Can I do anything to help these sailfins. Or have I just got the wrong fish together?

  7. Ogun says

    Hi, I have 4 tanks: 2 75 gallon tanks and two 10 gallons. I breed mollies, and they have been going for about 2 years now. I keep my tank at about 78 degrees, and I feed them pellets 3-4 times a day (very minimal in feeding). I found that the males are aggressive with other males until a champion is crowned, and then they chill out and mate with the females as much as possible. Then, when a new male surfaces, it challenges the reigning champion and if successful, takes over. It’s entertaining. One thing occurred that might sound crazy though, I had 3 black female mollies, 2 gold mollies (1 male 1 female), and 2 Dalmatian mollies (1 male, 1 female). After the two males fought, and at fist the Dalmatian one and had lots of reproduction, the gold molly grew up a little, toughened up, and then challenged again (they did this circular dance of nipping at each other with their top fin raised), and this time the Gold won. Afterwards, the gold bred a lot. In any event, after many frys, and after both of the males died, the three black mollies I had (which were considerably younger than the others at the time) grew just as large as the adults, and then, as if overnite, one of the females turned into a MALE!!! Is that normal? Can fish do that? At any rate, that tank is now dominated completely by black mollies, as they just took over. I keep a 10 gallon of just gold, and a 10 gallon of just silver, and both of my 75 gallon tanks are populated with all varieties. I keep adding golds and silvers to the other tanks, and I am hoping to get more diversity from my all black tank. But my main question is whether or not mollies actually change sex.

    • says

      I’ve heard that some saltwater fish change gender before, but I’ve never heard of mollies changing gender. After you posted your question, I did a bit of research, and it seems that some people have reported their mollies changing gender, but there are no scholarly articles on it. It seems the consensus is that mollies can’t change gender, but I can’t say they can’t for sure.

  8. Jake says

    Hi I have had 2 mollies die ( 1 make 1 female ) and I bristle nose all close together my female Molly was swimming with its head pointing to the bottom of the tank the tank had salt in it and the male disappeared over night MABY from the cat!!! What was the cause of the female doing that???

    • says

      I’ll need a bit more information. How large is the tank? Have you tested your water (most pet stores do this for free for you)? Do they appear to be sick in any other way – like a change in colour or white specs on them? Have you added anything to the tank lately, or changed anything else?

      • Jake Welch says

        No I have a 28 l tank and I have not changed the tank and when I had the other Molly but I had a smaller tank then

        • says

          It sounds like it might be an illness or the water quality may be off. You should consider doing a big water change. Does the molly seem bloated at all?

          And while the cat may be to blame, it’s more likely that it died and was eaten by the other fish. It’s nasty, but fish can be eaten very quickly after they die by their tankmates.

      • Nik Emery says

        Kollies that have been over and interbreed can have swim bladder issues. We have a white female who swells up every now and then and cannot swim normally. Some fish die of this as they cannot swim and breathe properly. Keep an eye on them. If it happens often you may have to euthanize your fish by putting him or her in a cup of tank water in the freezer. It’s painless as the fish simply goes to sleep then dies. Do not flush your fish as it is a $3-4,000 fine is caught.

  9. Ellen Cusack says

    Hi: I have a 20 gallon tank and I just started off two weeks ago with 2 female & 1 male Black Molly and 4 baby’s. One of the females died, I thought she had been laying eggs but I have since found out they have live babies. Anyway looking back I think the move and the male may have been just a little too much for her. I had the water tested and it was fine. The four babies were doing great I always saw them swimming around and they didn’t seem to be too intimidated by the adult Molly’s but they would get out of their way when it was feeding time or if they came too close. One week after I got the Molly’s I got 6 Neon Tetras and ever since then the Molly babies have taken to hiding and I think I may only have 2 left. They stay pretty close to the bottom gravel now and I can’t spot them very easily.
    I have a couple of questions. How long is the gestation period for a female Molly and how do you know when they are about to give birth?
    Thanks for any help you can give.

    • says

      The gestation period for a female molly is just under one month – usually around 28 days. You won’t be able to tell exactly when they are about to give birth, but they should be getting enormous when they are close to giving birth.

      As for the neons, their mouths should be too small to eat the molly fry, but the male molly may be eating a few. Most fry will generally hide from adult fish, which is why it’s always a good ideas to have floating plants in a live-bearer tank.

      Also, with only one female in the tank now, the male will relentlessly mate with her, so you should try to get at least another molly female in there if you can.

  10. sherry says

    HI, im slighty new to fish. I believe i have mollies (6) 4 other fish that im not sure what they are and one tiger barb i believe is the name. One of my molly fish seems to be getting a little large and is becoming more aggressive during feeding time. Could this be a she? and could she be pregnant? i currently have a plant that is rooted but floats to the top like a lily pad, but i was told i need a plant that primarily floats what do you recommend?

  11. Tim says

    I have a 55 gallon tank about 3 weeks old. I bought 3 dalmatian mollies and put in after a week. 4 days later one of them kept sinking to bottom and died before I could treat with antibiotics. I then bought 6 neon tetras an aquatic frog and algae eater not chinese. I did not quarantine them first just added them. I’ve lost 3 tetras and had another molly have similar symptoms as the first. So I moved it and treated it for thre idease days. Putting it on a fast as I read it may have ate to many flakes and they swell inside causing pressure on bladder. Today I reintroduced it and still seems to stay low. I added salt the second week and other than av hard water the test show my ph is alkaline. Have not tested for salinity but I put in less than suggested. All other fish are fine. Any ideas

    • says

      It sounds like you’re doing most things right, and it most likely is a bacterial or parasitical infection. You should continue treatment with medication, but be sure to remove any charcoal from your filter as it will remove all the medication from the water.

      Also, at the end of the fast you should feed shelled peas and if your fish gets very ill, you can lower the water level and try feeding it by hand. You can also add plants at any stage in the aquarium, and they will actually help with the water quality at first. – which is very important when your fish are ill.

  12. Jennifer Murphy says

    This has been so helpful. We currently have four adult Mollys. One large male that we’ve had for about a year and another female that we’ve had for a while as well. We had another Dalmatian female that gave birth and died.Two of her babies survived and are now a few months old (both male.) We also now have 8 babies in the tank. I am trying to find homes for the babies once they grow since our tank cannot hold that many. The large adult male is very aggressive. He chases the female around the tank constantly. My son is very attached to him though as he’s his oldest fish and even made it through a hurricane. We do not want any more babies. If we were to find a new home for the female, would it be OK to have the three males together? Thanks!

    • says

      The three males should be fine together. They may get a little bit aggressive towards each other, but they won’t constantly bother each other like they would a female. With three males in the tank she must be getting very little in the way of peace.

      If the males become a problem, just rearrange the aquarium a bit so that there are more hiding places, and try to arrange the decorations so that they can’t see the whole tank. With most fish, once something is out of sight, it’s out of mind.

    • says

      I don’t really like breeding nets. They work fine for small fish like guppies, but I find that they really stress out larger fish like mollies and platies.

      They’re fine though if you can catch the fry and put them in a breeding net after they’re born. But a lot of it is personal preference too.

  13. Sara B says

    Hi, I have only very recently bought a 64l tank…..added 6 neons and 6 rasboras the first week…..then a week later added 2 Mollies. I did not know at the time how to tell whether they were male or female, but they appear to be one of each! Anyway the female must have been pregnant already, because when we came home today we found a tank full of fry! Approximately 30! The Dalmatian Molly is now half the size she was! We were told they were balloon Mollies, hence the bulbous bellies! hmm I think that would have been a pregnant belly! lol As for getting on she seems to flirt in front of him more than him chasing her! But I am sure he will chase when he is ready!

    Anyway what can I do to give the fry the best chance of survival, I have bought another tank 21 litre, filled it with water from the 64l tank, put live plants, filter, heater etc and transferred all the fry! (which was a challenge, took hubby and I about 2 hours with a net! anything else I can do to give them a fighting chance?

    • says

      Fry can be notoriously difficult to catch – I feel your pain on that one. (I’ve destroyed plants setups trying to catch them in the past.)

      But the most important thing for fry to survive is pristine water quality. You have to do something to cycle the tank if you want them to survive. The live plants are good start, since they will help with the water quality, but if possible you should set up a filter with media from your other filter.

      Depending on the type of filter that you have, you cut a small piece of the sponge or filter insert, and place it in the filter on the fry tank. Failing that, you can take a handful of gravel from your cycled tank and place it in your fry tank. The gravel should also carry some of the needed beneficial bacteria, and will help the bacteria in the tank become established.

      It also helps with fry if you can feed them several times a day, in very small quantities. Try to feed them in the morning, after work and once shortly before you turn off the lights at night. Make sure that if you do this, that the food is kept to a minimum, especially in an uncycled tank.

      Hope this helps.

  14. Sara B says

    Thank-you, I have scooped some gravel out and put it straight in, the filter is fixed on a plastic frame, so can’t use some of that unfortunately.
    I hope they do well! I feel like they are my babies! ha ha, sad isn’t it!

  15. Lauren W says

    Hi, my Creamsicle Lyretail Molly (female) has had 2 groups of fry one that is 6 weeks old now (consisting of 34 fry) then she had another set which are 6 days old (10 fry). I had a Dalmation Molly (male) in the tank with her but have moved him to another tank. I was told by an employee at a pet store that people put the fry in outdoor ponds. I wanted to know what you thought about that ? I would love to keep them all but I already have 3 fish tanks that cannot handle all of the 44 fry. I was thinking of getting another tank just for them but I am afraid of them breeding and I do not want anymore fry. I am not sure of their growth rate and how long it will be before I need to move them into an even bigger tank if they all survived.

    • says

      Putting fry in a small pond outside is a double edged sword. On the one hand, the natural food that they find outside will help them to grow extremely fast and they will often be much healthier. The downside is that being outsides puts them at risk of predation, and it’s not unusual for predators to empty a small pond in a matter of days.

      If you really wanted to keep them, you should probably keep them inside. And they will keep breeding, and breeding, and breeding… lol. I spend a lot of time trying to get rid of all my different fry, so it gets to be a bit much at times.

  16. says

    Hi Matthew,
    I’ve a 30 gallon tank with a mixture of 8 guppies, 4 neon tetras, 3 mollie females and 1 sailfin male Molly we affectionately named Elvis – due to all his posturing around his lady fans.
    Stats: 80 degrees, occaisionally add salt, change water by 10% every week, Nitrate 0 / Nitrate 0.5, ph 7
    Here’s the problem:
    I had a sick guppy 2-3 weeks ago with a parasite – diagnosed by a very long, stringy white poop. I had quarantined her for 3 weeks, but this happened after she was added to my main tank.
    My favorite aquarium store recommended Quick Cure – formalin and malachite green. A course of Tetracycline didn’t seem to do anything.
    So I dosed at 50% the recommendation on the package because of my neons (after removing the charcoal).
    Lost the guppy (sad).
    But then, my Sailfin Male Molly got a hole in his side right behind his side fine, and stopped swimming. Started hiding in the corner and not eating much.
    Moved the tetras into the smaller quarantine tank, so I could treat the 30 gal tank with the full recommended dose of Quick Cure.
    Did the 25% recommended water change and treated for another 3 days with Quick Cure as per package.
    Here we are, a week or 2 into this process, and now I see that, while the hole in his side is somewhat healed and he’s at least swimming a little, he’s got cottony growths on the same side and now he is trailing a 3″ stringy white poop like the guppy that died.
    He won’t touch the treats – bloodworms, brine shrimp! nope, nor the flaked food. All he’s a tiny bit interested in is Omega One Veggie Rounds, and he just plays with that, really.
    Matthew, I’m really worried for my whole fish family now, and don’t know what to do for Elvis!
    I have pictures but don’t see where to upload them.

    • says

      Hmmm…formalin and malachite green are mainly used for external infections, and malachite green is really hard on the fish. And in the past I’ve found that it kills fish faster than the infections, but that’s just my opinion.

      The malachite green should treat the fuzz, which is likely fungus. If it doesn’t, than it might be columnaris which requires antibiotics. The key thing you need to deal with is the internal parasites, once the quick cure dosing has been completed.

      All of the different treatments will wear down the fish, so after you complete the quick cure, you should make sure you put your charcoal back in the filter to remove any left in your water. You should do a large water change, and then use a anti parasite medication. There are many good ones, so see which ones are in your area.

      You can send the pictures to

      I hope that your fish start to recover and once I see the pictures I can probably help more.

  17. Kibee says

    I bought 3 creamsicle mollies together. Didn’t ask what sex they were and the info wasn’t volunteered at the pet store. I late found out that I have two males and one female. After a little research online I found that the two makes could pester the female to death so I bought another female, this time I got a dalmatian molly. Problem is the creamsicle mollies. Problem is the creamsicle mollies won’t interact with the dalmatian. I only have a 10 gallon track and am very hesitant to add another fish. Any advice is helpful.

    • says

      That’s odd, as I’ve had many different species of mollies interact over the years. And you definitely don’t want to add any more fish, as the water could crash very easily if you do.

      I would just give it time for now. Sometimes they take a while to accept a new addition. Are they close the same size? Could they be different species? If some are sailfins and the others shortfins, that might be your reason too.

  18. Rachael says

    I was wondering if you could please offer some advice. We have a ‘banishment’ tank if you will. It has a black angel fish, a huge goldfish/comet, a silver catfish sucker, a mini orange and black molly (f) and recently a beautiful large sailfin molly (m). Now as soon as M molly was added to the tank (which is 60L, 7.0PH and 25*c) he immediately started to…. Rape the goldfish? Now Goldie is easily 6 inches long and Hustler (male molly) is about two inches. It was quite humerus to begin with that he was trying to in pregnant the folds gosh instead of the female molly, however Goldie has begun hiding under the filter instead of the usual parade around the tank. Is there any reason Hustler is basically raping a completely separate species more than 5 times his size?!

    • says

      It’s not as uncommon as you think. Since you already have a banishment tank, you already know that certain fish just don’t do well together. I’ve had mollies try to mate with all sorts of species and it gets a little ridiculous. The only thing to do is to add more plants or decorations that will help to break the line of sight of the molly.

      Other than that, there’s really no way to dissuade the molly from having fun (unless you want to add a few female mollies).

  19. julie beth says

    Is a 20 gallon tank big enough to make 3 female Mollies happy? It is highly planted and I test water parameters regularly. It’s newly empty and cycled, as I just transferred ‘the gang’ to another tank. I’m also curious how you feel about pot-belly mollies? I’ve heard some controversy over them, but they are indeed adorable and because they’re smaller, I was considering them. Any thoughts? Thanks!

    • says

      It depends on the species. Short fin molles, yes – sailfin, no. So just pick out a species that is smaller and you should be fine.

      As for the potbelly, my significant thinks they’re the cutest thing in the world, but personally I’m not sure about them. They’re still able to swim ok, but they are more prone to problems and they can have trouble with their intestines. The upside is that they’re not the monstrosities that many of the species of goldfish have become, and they’re not intentional injured like dyed fish. I think as long as you take extra good care of them, they are generally ok to keep. (I’m sure a lot of people would disagree with that). lol

  20. Parth says

    Hi Matthew i have 2 orange mollies one was pregnant from before only and other one is male and i also have 2 small fishes who has a colourful tale so according to my research mollies baies will hide but the tiny tots are not hiding they are just staying in a school and i have 2 shelves that is shankh u know so they are not going inside it it is covered and i have may hydras too so tell me something good and affordable for them they are by the way just born today!

    • says

      I doubt that you would have hydra in a tank with mollies, since they enjoy eating it so much. If you do have hydras though, you can sometimes remove all of the rocks and decorations, and put them in a 10% bleach solution. This will kill any hydras on them, and you can scrap any off on the glass. Make sure to thoroughly rinse anything before you put it back in the aquarium.

      As for the babies, they often form a school in the beginning but they will generally hide when a large fish comes near. You can add floating or dense plants if you want to give them more places to hide.

  21. Rachel says

    i have 5 bettas and two molly’s in a tank, and my molly’s bully each other and the bettas… to tell you the truth; my molly’s are bullying my bettas mercilessly. Also one of my molly’s is growing a long thick string behind his first fin it’s, I think, a beige. Please help.

    • says

      How big is your tank? As for the string, are you sure it isn’t part of the betta that has been damaged? Like has the fin been ripped?

      The problem is a set up like what you have, is it’s going to cause a lot of aggression. The only hope is to rehome some of the fish, or at least try to create breaks in their sight lines.

      • Rachel says

        It’s a 10 gallon tank, I think it might just be poop now that I’ve researched it, it’s gone away. K, I’ll try to rehome Bumper.

  22. Mindee says

    Hi Matthew, I’ve really enjoyed reading this page! Great info! I have a 20 gal tank with 1 fancy goldfish & 1 clown pleco. I’d really like to add some other fish, but I’m not sure what kind or how many. I do not want to breed anything! I know I shouldn’t add anything with the goldfish, but is there any other breeds that would be alittle compatible?

    • says

      I’m glad that you’re enjoying the website, and you’re going to hate hearing it, but I wouldn’t add anymore fish to that aquarium. Goldfish grow huge and are a coldwater fish. If they are kept in water that is too warm, they can suffer nerve damage and have shortened life spans. Clown plecos on the other hand are warm water fish, and while I don’t know for sure if clown plecos do this (I’ve never kept them personally), I know that many plecos will attack goldfish and suck off their slime coat.

      If you wanted to just keep a coldwater tank, you can keep goldfish with rosy red minnows, zebra danios, gold barbs, weather loaches, and white cloud minnows. Most of these won’t do well in a true cold water tank, but should be fine at room temperature. Also, the goldfish will eat anything that will fit into their mouths, so this could be a problem as the goldfish grow. Some of these fish are fin-nippers too, so if you have a comet of fancy goldfish, you need to be careful.

  23. says

    Hi again Matthew,
    Love this forum – I’m learning so much!

    I’ve several tanks – the one in question is a 30 gallon tank with a mixture of 7 guppies, 4 neon tetras, 2 black molly females, 1 male dalmation molly (the gold male unfortunately passed away), & their fry (4 @ 1/2″, maybe 6 @ 1/4″ and probably a dozen at 1/8.” I give the fry away once they get to about 1/2″; I just find it fascinating watching the babies grow!

    Stats: 78 degrees, change water 10% every week, Nitrate 0, Nitrate 0.5, ph 7, hard water (GH 250), moderately planted tank. HOB filter changed monthly. I feed lightly 3x a day; I mix it up every meal between Omega 1 freshwater flakes or veggie rounds, Tetra veggie flakes, API tropical flakes, frozen: bloodworms/daphnea/baby brine shrimp. The light is a plant bulb – it’s on for about 12 hours a day, and the tank gets about 1/2 hour of morning sun around 7-8 am.

    Here’s the question: Most of the molly fry are neither black nor gold, but sort of pinkish yellow with gray markings. The older molly female has gone from all black to practically all gray over the past several months, during which time she’s had 3 sets of fry. The local fish stores have no idea why, perhaps pregnancy hormones? After noticing the silver molly female was chasing her constantly, I put the silver into a different tank to see if the pigment loss was stress, but she’s only lost more color. Should I move the black molly female into another tank without any males? One more thing – the other day I noticed the dalmation molly is starting to get brilliant blue speckling.

  24. caitlin says

    Hi there, I have a female black molly that just last night gave birth(?) to 15 fry. They were all born with these orange orbs attached to their bellies, it looks like their stomachs are either super stretched out or as if the orange orbs are their eggs that are attached to them. I have honestly no clue what I am doing.
    We bought her and she turned out to be pregnant. Please help, out of the 15 there are only 5 living as of this morning. I have no clue what to do. I feel so bad, these poor babies kept dying off, are they in pain?? I have them in the breeding chamber and put in flakes ground up really small. I have kept the lamp on by them. What should I do?

    • says

      Don’t worry about the bellies – they always look like that with newborn fry. Their bellys always look big, and may be the only part of the fish that really has any colour.

      I’m not sure why they are dying off though. One of the reasons that many people raise them in their own tanks, is that they are very sensitive to water quality. And with other adult fish in the tank, it can be hard to keep the water quality perfect like they need.

      The only thing to do, is do a water change, and feed them very small amounts at least twice a day. Grind up the flake food so they can eat it. Hopefully a few of the strong ones will make it through.

  25. flicker says

    Hi, I have a 70L tank with 3 mollies (plus others). I got my mollies from a specialist fish farm who advised my two gold and blacks are female and the dalmatian is a male however in the past 3 days one if my gold and blacks has developed that anal fin thing you have mentioned previously – the dalmatian always had it but I swear this one didn’t previously! You have mentioned their sex developes as they age however all 3 are of a good size (at least 2 inches long if not 3). Could this be the cause? Don’t really want to get any more due to over stocking the tank but will if I must. ..

    • says

      That’s odd that it seems to be changing. Its common for them to be hard to sex when they are young, but I’ve never heard about that in mature mollies.

      I would just monitor it for it a bit and see what happens before you overstock it. Its rare, but sometimes I’ve seen anal fin injuries which make it look different.

      Have the mating attempts changed?

      • flicker says

        The dalmatian has been chasing the ‘new’ male around with a raised back fin. The ‘new’ male has been flicking his male fin thing forwards sometimes. Apart from that as a first time fish owner I’m not sure what the mating habits are :-)
        The one thing I have noticed is the ‘new’ male is a more vibrant colour then the female. Again only noticeable recently. The dalmatian has always been aggressive but the back fin is new

  26. Julie says

    Hello, we have recently purchased 6 Lyretail Mollies, we also have 12 Harlequin Tetras in the tank. The tank is 220 litres. We would like to add some tetras but have read that they can nip the fins of the lyretails. Any suggestions as to what tetras we can keep with the Lyretails/ Many thanks,,,

    • says

      I’ve always like Harlequin rasboras. While they aren’t a tetra, they look similar and are an extremely peaceful fish. If you’re only only looking for a tetra, then I’ve heard good things about cardinal tetras and rummynose tetras. I’m only giving that information second hand, since I generally stay away from barbs and tetras with my mollys. Also, my water is really hard, so aside from blind cave tetras, most tetras would just about melt in my water. lol

  27. Michelle says

    Hi, I hope you can help me. I have 1 female dalmation molly and 1 male black lyretail molly. I was hoping to breed them but I just recently found out that male lyretail mollies are usually sterile. Is this true? And also, the female molly keeps bullying the male. He is slightly smaller than her, but I thought the males were the dominant ones. Please help! Thanks

    • says

      I’ve never bred lyretail mollies myself, but I do know that most lyretail mollies are sterile. Apparently the gonopodium is too long to function on many of the males, so it is very difficult to breed them. Also, generally the males are dominant, but they will act asexual sometimes if their is another male in the tank. Could the female actually be a male? Otherwise, it may just be temporary and should cease as he grows. Some male mollys are late bloomers.

      • Michelle says

        Nope, the dalmation molly is definitely a female. She has the rounded anal fin. And before I got the male black lyretail molly, I had another female black molly in the tank. I thought she was going to have fry because she looked pregnant, but the dalmation molly kept bullying her so much that she ended up dying from the stress I guess. So I thought I would get a male to chill her out and thought he would take over the tank, but she is bullying him too. If he is a late bloomer, them i guess she wouldnt feel threatened by him yet. I don’t know. I’m so confused by her behavior. Maybe I just need to get a dalmation male molly and see what she does.

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