Molly Fish Care
Updated May 14, 2020
Author: Mike - FishLore Admin
The Molly is a tropical fish that prefers some salt in their water. A teaspoon of aquarium salt per 5 gallons of water will go a long way in helping them. You may also see them in saltwater tanks from time to time. There is a local reef store here that keeps black mollies in a regular saltwater tank with similar sized species.
The molly is a very attractive tropical fish that comes in many different colors such as orange, green and black. Some of the more popular varieties include the sailfin, balloon and the dalmation.
This is a livebearing tropical fish that can be fairly easy to breed. For many aquarists the biggest problem is not getting them to breed but stopping them from breeding. If left in a tank with other adult fish, the babies will get eaten.
They will eat flakes, frozen, freeze dried and definitely live fish foods.
Regarding Balloon Mollies; the balloon trait is obtained by selectively breeding fish with scoliosis (bent spine). Balloon mollies often have a reduced lifespan due to three factors: the inefficiency of the misshapen morphology causing wear and tear while swimming, the compact position of its internal organs in comparison to a non-balloon variety, and often seen difficulties giving birth.
To salt or not to salt?
Sailfin and black mollies can tolerate a little bit of aquarium salt (mexican mollies require it), up to about 1 table spoon of salt per gallon. It has been argued, however, that salt actually irritates their skin. I myself only use salt in cases of ich. If you do use salt, please use aquarium salt and not kitchen salt (has natrium in it) or marine salt (has minerals in it).
Molly Fish Care Details
Scientific Names : Poecilia sphenops, P. velifera, P. latipinna and many others.
There are 3 different species of mollies, which have been re-classified, ungrouped and re-grouped several times. Most of the variations you see today are man-made. The current consensus on the main species is as follows:
Peocilia velifera (also called P. mexicana) = Yucatan molly. Sometimes erroneously referred to as sailfin.
- Size: Larger than P. latipina (about 8cm/6in for big specimens).
- Dorsal fin: Males' dorsal fin is high. It has more rays (about 18) than the latipinna. Also, upper edge of dorsal longer than lower. Dorsal fin markings are slight, small spots.
- Salt: really need brackish water to thrive.
- Other requirements: They need direct sunlight, and more gallons per fish than the latipinna.
Peocilia latipinna ('true' sailfin).
- Size: Smaller than velifera (about 6cm/3in for big ones) and more slender
- Salt: Tolerate some salinity but do not require it.
- Dorsal fin: The male's dorsal fin is high. has less rays (about 15). Also, upper edge shorter than lower edge. Their dorsal fin is edged in a yellow line and they have dark and square markings on the dorsal fin.
Peocilia sphenops (short-fin, e.g. black molly).
- Size: Smaller yet than the latipinna (about 6cm/3in for big males; 5in for big females) and more slender
- Salt: Tolerate some salinity but do not necessarily require it.
- Dorsal fin: Their dorsal fin is smaller than either sailfin type.
- Not only colored in black, but also mottled as well as a gold variety.
Some morphological varieties have been bred into the three species: you can now find balloon (pot-belly) and lyretail varieties, for example, and a whole sort of color assortments.
Common Names : Lots of different types are available: Black, Lyretail, Sailfin, Liberty, Mollie, Pointed Mouth, Short Finned, Mexican, Golden, Piebald, Ghost Pearl, Gold Dust, Red Sunset, Dalmation, Ballon, etc.
Care Level : Easy and a very good for beginners if keeping a single sex. Keeping both sexes could cause problems since this fish is such a prolific breeder.
Size : 2 to 4 inches (5 - 10 cm) depending on the species
pH : 7.5 - 8.5
Temperature : 70°F - 82°F (21°C - 28°C)
Water Hardness : 10° to 25° dH
Origin / Habitat : Central America
Life Span : 3 - 5 years
Temperament / Behavior : Peaceful
Do you have a pregnant molly? Read the breeding guppies article for more information. That article talks about guppies and swordtails, but the process is very similar for the babies.
If you want to try and raise the fry think about getting a breeding box or breeder net like these:
Frequently asked questions and answers on breeding them:
How can I tell if it's a male or a female?
The male has a pointy-shaped anal fin (gonopodium), whereas the female has a 'normal' triangular set of anal fins. For sailfins, the males also have a high dorsal fin.
How do I know they're mating?
The male will constantly chase the female and point his gonopodium towards her belly in the chase.
How long is the pregnancy?
Normally speaking, the gestation period is abour 4 weeks after mating. They can get pregnant immediately after giving birth, so it's not uncommon to have your females nearly PERMANENTLY pregnant, and giving birth every 4 weeks or so.
Females can hold sperm without letting it develop into fry for a very long time (weeks and weeks on end). They'll do that if they feel the conditions are not right for their young to develop. This is causing the oft-seen phenomenon of females arriving pregnant form the shop, prompting many people to bewonder the miracle of immaculate conception, or extreme cross-species breeding with, say, blue whale tank mates.
What's a livebearer?
They are livebearers, which means that the fry is born free-swimming. They are ovoviviparous, though the young does have an egg stage inside the mother, unlike mammals, which are viviparous (no egg stage at all: the baby IS the egg).
How many fry will she have?
A young female's first ever batch can be as little as a dozen fry. As she gets pregnant again (and again, and again and again...), she's likely to give birth to increasingly large batches. Some people report prolific mothers regularly give birth to 80 or so fry per batch. They may give birth in 2 or more phases, releasing the second half a week or so after the first batch was dropped.
How do I know she's pregnant?
A female about to give birth will look almost boxy in shape, distended that she is with all the fry inside her. Another tell-tale sign that she's about to give birth is that she may lack appetite a day or two before dropping her fry. She'll also likely isolate herself form the others, and lay in a quiet spot on the substrate. This behaviour often prompts panicked owners to diagnose a disease, but it's common in pregnant livebearers, and she'll soon recover her spirirts. If your fish is relatively transparent in color, you may distinguish what is called the gravid spot on her abdomen. This looks like a darker spot, and is the mass of fry that you can see through the fish. The closer she is to dropping, the closer the spot will be to her cloaca.
Why does my fry disappear?
They (and other tankmates) can eat their young. The young also commonly get trapped under gravel. If you want to keep your fry alive, you'll need to either provide plenty of cover for them (a floating plant, mini pots/jars on the substrate, stacked flat marbles or a densely planted area should ensure that some of your fry makes it to adulthood) or to isolate the female prior to her giving birth. I personally never isolate my pregnant females, as it stresses them out. Simply speaking, the fry cease to be in danger when they are too big to fit in the tank mates' mouth.
When do I know if the fry are male or female?
You can sex the young from about 3 months old. Before that, the immature fry will all look like females. Do separate them from their parents before they reach maturity to avoid inbreeding.
Interbreeding between species
All three molly species can interbreed. Other livebearers can also breed with mollies. The following interbreeding combinations have been reported: guppy-molly, and swordtail-platys. But beware, the resulting fry are mostly sterile, deformed runts, and you will not be able to give them away to a shop.
If you don't want fry all the time, you'll need to separate the males from the females. Same-gender tanks should really have at least 3 together to avoid the alpha fish victimising the subservient fish. Mixed-gender tanks should keep at least 1 male per two, or even better, three, females. You'll soon notice that the male's insatiable appetite would end up exhausting a lone female.
Aquarium Size : 20 gallon minium, prefer tall aquariums
Tank Mates : Not many - some feel they should only be kept with others of the same species.
Fish Disease : Freshwater Fish Disease - Diagnose, Symptoms and Treatment
They can show the following disease symptoms:
Shimmy: get shimmy block or molly bright medication to catch at onset. Shimmy is sign of early development of one of several possible diseases.
Torn fins: either result of a fight or finrot. I would isolate the fish and treat with disinfectant. If it is finrot, you will need to determine whether it bacterial or fungal before treating.
Flashing/scratching: water quality problem (nitrate/nitrite/ammonia) likeliest. External skin parasites also likely.
Lethargy: commonest cause is just pregnancy.
Gasping for air on the surface: temperature could be too high. oxygenation could be too low. could be nitrate/nitrite/ammonia poisoning.
Reddened/purple gills: sign of nitrites/nitrates/ammonia poisoning.
Dull white film on skin: could be oodinium (turn the tank light off and shine a torch on the fish. If the white patch glows back, it's oodinium). Could be a fungal infection (furry-looking white patch, like mold).
Slanted swimming, swimming upright or on the side: possibly a swim bladder problem, or a spine problem. Spine problems can be caused by a vitamin defficiency or by TB.
Pop eye: Symptom of many possible diseases, including the dreaded fish TB. Could also be mechanical damage from fight or knocking against ornament. Pop eye can also be caused by water pollution with plastic or metal, or by ammonia/nitrite/nitrate. In rare cases, pop eye is caused by oxygen oversaturation (gas bubble disease).
Dropsy: (means fish's midsection is extremely swollen, and scales stand upright in worst cases). Symptom of many possible diseases, including fish TB.
Sunken belly: Symptom of many possible diseases, including fish TB. Sometimes the fish has trouble eating from a wound around the mouth area.
Grain-like white spots: Ich.
Constipation: Poo constantly stringing out of fish, or fish's belly really distended.
Stringy excrement: internal parasites.
Diet / Foods : Give them a varied diet consisting primarily of flake foods but supplement with live and freeze dried foods occasionaly. Some feeding tips:
- Slow-sinking protein and vegetable pellets, served on the surface so they get to chase stuff
- A variety of vitamin packed flake food promoting growth, color and immune system
- Fast-sinking protein and vegetable pellets so they have to browse for their food
How Much To Feed Them?
A common rule of thumb is that a fish' stomach is the same size is one of its eyes, so try to stick to one stomach's worth of food per fish per feeding. Feed in slow batches, giving a small portion and waiting until they've eaten it before giving more. Fasting them one day a week is a good idea too.
Tank Region : All over the tank.
Gender : Males are more slender, females more round. Males also have a modified anal fin.
Photo Credit : Photos copyright JJPhoto.dk
Molly Tips and Comments
How often do they breed and can you stop them if you want to?
I moved my female today to a breeding tank, I think she might have been too stressed and aborted the pregnancy. How can I tell if she did?
Mine had about 39 babies and as they grew I couldn't tell the sex of them. I knew the females where more round but does the male have a larger top fin?
On average how many eggs do females lay in one pregancy?
How long do they live for?
How long does the pregnancy last until the babies are born?
My fish had babies and we put them in a separate tank with the mother and she ate her babies. Is this normal behaviour of females?
I've been breeding livebearers since I was about 12. I've tried breeding betta fish, but the babies are so small and fragile that they always die. I took a long break from livebearers and had some tetras and angelfish. I had a Dalmation Molly that had 14 babies and only one made it to this day. The mom and dad got eaten and I moved the baby one to its own tank immediately. I just bought him an all black girl and she just had 9 babies two days ago! Black babies are the absolute cutest! The 2 adults never picked on them but I moved the parents to a larger tank and bought 2 gold dust mollies and a male swordtail. Breeding this fish is the coolest thing in the world. They are so adorable and is a hobby of mine.
Yesterday we bought a white, black and two spotted ones. We woke up this morning and our white molly was dead and now there are about 15 to 20 babies! We had no idea this was going to happen it was a shock to say the least. The others haven't been interested in eating the babies, but we will see.
Mollies are really easy breeders. The molly will produce a lot of babies if you minimize the time the light is on to about 13 hours a day max.
I read you should put some aquarium salt in the tank to help mollies fluorish. Can you use table salt?
Baby mollies are fun to raise, we have just started. We only got two babies out of the first batch. Kids really love to watch them grow. Females like the Krill the best I have seen.
I've been raising mollies and platys for a few years. They are really easy to bread. Just Keep the water clean (change 20 to 25% every week or two). Get a net breading area for the fry. Give the fry finely ground floating food. Don't leave the adult mother in the breading area long after birth or yes, she will start eating the young. If you don't want a large population of fry just leave them in the tank and provide hiding areas for the fry. Cambodia grass works very well and depending on your adult population, about 10 to 20% of the fry will flourish. Good Luck and enjoy.
I have been keeping mollies on and off for years and have never known fry to be eaten by other mollies! We have just raised a batch of 70 or so fry in a tank with their mother and 4 other adult mollies - perhaps we have been lucky, but I also feel that feeding all the fish small amounts at regular intervals goes a long way to avoiding the adults taking the fry. These great little fish are so easy and such a joy - they make amateur geneticists out of all of us with their willingness to breed!
I have a 5 1/2 year old female Molly, Raven and one thing that I have found out is that Mollies seem to be very social with other fish. There are three other different fish breeds with her in the aquarium and she is very well mannered. I also added a male betta, with my Molly and other fish and they all get along very well. Mollies are a very social fish I have found.
I started with two mollies, both female, and as it turns out, both pregnant. They started in a 15 gallon tank with a small shoal of black neon tetras. Now I have close to 30. I moved them to a tall 40 gallon tank, and when I added salt to fight an ick breakout, they went wild.
I woke up one morning to 5 new baby fish. I have no idea who the mother is. I didn't know any of my fish were pregnant. I have them in a breeding container now, and they are just the cutest things. I feed them ground up flake food. I place the food in a plastic baggy, grind it up, then dip one end of a toothpick into the tank water, and then into the powdered food. I then dip that end back into the water right in front of the babies and they gobble it up! All I need to know now is who their mother is? I have 2 white mollies, 1 orange molly, and 1 dalmation molly. I wonder what the babies will look like? I have more fish in this aquarium as well: angel fish, cat fish, bettas, both male and female, tetras, kissing fish... It remains a mystery! But a fun one!
I bought 2 dalmation mollies, and 2 black mollies not long ago and the dalmations had babies one day, then the black ones the next. I also have two platies and a couple of neon tetras and none of them bother the babies at all... My fish don't seem interested in eating their babies.
I have two molly fish one is the black and the other is the gold dust and they're both females. I was wondering if they need a light on them all the time or could you turn the fish tank light off at night?
I bought my two mollies and within 2 weeks she was putting on weight. A few weeks later she's popped out 20 or so little fish! I'm not having any problems with the adults eating them. Live plants and good circulation keeps them happy and the eco system healthy. The babies have been fine eating normal fish flakes, but I'm going to try the whole "crush in a plastic bag" trick to sort of break it down a bit.
I had 35 fry black mollies, 20 died now I have 15 fries left. I feed my fry 2 times a day a little bit! Some of the flake food goes to the bottom and my fry eat it happily! But soon they will get bigger and you will have to get a bigger tank! And you will have to feed them bloodworms, microworms and stuff like that. You can get this staff at Big Al's tropical fish store and you can buy a book there about other types of mollies and other tropical fishes! Fry are fun at first but then a couple of times later it wouldn't be so much fun anymore!
Related Fish Profiles
Another popular freshwater aquarium fish species and a very hardy livebearer. They come if tons of different varieties.
Many colorful varieties are available and males develop a signature "sword" like tail.