Molly fish is one of the best fishes for beginners

Did you know that molly genus (in Latin – Mollienesia) was abolished and now all fishes from it are included into livebearers genus (in Latin Poecilia). Well, I didn’t. I have found it only when preparing this article. And the fact, that various fishes that we know as molly fishes are now also in this genus only adds more confusion.

I’ve tried to puzzle out all this stuff, but I failed. White molly fish, black molly, yucatan molly, sphenops, sailfin molly — whatever fish you may name – all of them are popular and wide spread as tank dwellers, though quite often they look very differently.

All this is due to their peacefulness, endurance, unpretentiousness, inexpensiveness and easy breeding.

This fish is the best choice for beginner aquarists – common molly fish. It is less demanding, easier to breed and requires smaller tank capacity. In general, in this article we’ll talk about molly, though almost all we say will work for the other species.

All fishes differ only in size, but the info about breeding, keeping, compatibility is the same for all of them.

Habitat in the wild

Mollies habitat is rather diversified – freshwater, brackish and saline water of different temperature, hardness and acidity. Various Poecilia fishes have some differences in their morphological characters, they’ve got used to different life conditions.

Sphenops is encountered in waters of Mexico and Columbia, sailfin can be found in channels with brackish water in state of Carolina, Texas, Virginia and Florida, yucatan molly can be seen only in Mexican peninsula Yucatan.


The fish has an elongated body a bit flattened from sides. Its eyes and upward mouth are relatively large. The male grows up to 3 inches long (6–7 cm) and the female is about 4 inches large (10 cm) at the age of about 6 months.

The beautiful fins get their maximum beauty and size only during the 2nd year of the fish life.

Reference samples of black molly fish don’t have any glitter or spots on their body – they are coal-black.

Keeping in a tank

Scientific NamePoecilia
Common NameMolly, molly fish
Tank size10 gallons and more
Temperature72°F- 78°F (22 to 26 °C)
Sizeabout 4 inches large (10 cm)

In my opinion minimum tank size for successful keeping of small group of mollies is 13 US gallons (50 liters). But these are active fishes, therefore the perfect tank size for keeping them will be 26 US gallons (100 liters).

Don’t keep the fish in small and crowded tanks. They may grow to become 10 cm long and will not feel comfortable in a tank of small volume.

There should be a stable biotic balance in a tank. All fishes are quite thermophilic and they prefer water temperature about 25- 27 °C, they don’t stand rapid temperature changes.

Parameters of the tank water may be quite various, since fish adapts to any new conditions perfectly. However, the following parameters are recommended: water temperature 25- 27 °C, ph: 7.0-8.0, water hardness 20 – 30 dGH.

We should also mention that molly can live in brackish water as well and in some resources they even recommend to put some salt into the tank water intentionally.

Well, this will not harm the fish, but don’t forget that this fish quite seldom inhabits without any tank mates and the latter may not stand brackish water or it could even be toxic for them.

Therefore, I’d recommend to add some salt into the tank only if mollies live alone or when they are in quarantine.

Just like all other fishes, molly likes fresh water, that’s why it’s desirable to renew water systematically. Once a week perform water renew in the volume of ¼ from the whole tank capacity.

This will ensure absence of high concentration of nitrogenous compounds in the tank, such as ammonia, nitrites and nitrates.

Water filtration and aeration are required. It’s better if the tank is a thick planted one, because molly fish like scrapping algae from tank plants.

Most of the time the fish spends in upper and middle water layers. You also have to create open and well lighted areas in the tank. Snags, stones, lots of plants should be present in the tank as well to provide the fish with some shelter, if needed.

Compatibility and tank mates

Mollies is a very active one and it is considered as a peaceful one, though I’d rather say that it is a moderately aggressive one. Yes, in a roomy tank it behaves very peaceful and calmly, but if the tank is small and crowded the fish can show aggression towards its kind and sometimes to other fishes in the tank.

It’s better to keep molly in a group, where females prevail. This is a common rule for all live-breeders, since when the number of male is equal or higher than that of the female fish, the latter will be always stressed due to the males haunting them all the time.

The fish can live both in a common tank and separately. It gets on well with various tank mates. In general, tank mates have to be approximately of the similar size.

Molly have good relationships within their group – they almost don’t show any intraspecific aggression.

But at the same time, if the tank is small the males may haunt each other. That’s why it’s better to have a spacious tank, so that at least 2 gallons (10 liters) of water would fall for one fish.

Mollies get along well with peaceful and small fishes like dwarf gourami, bristlenose pleco, white cloud mountain minnow, harlequin rasbora, cherry barb, guppy and platy.


This is an omnivorous fish that eats all types of live, frozen or artificial food. However, fish needs large amount of food with vegetable fibers component, for example, with algae or vegetables.

The thing is, that in the wild fish has lots of algae in its diet, you can see it from the lips and behavior. You can often see how it scrapes algae and fouling from tank glass and decorations.

As for the food with vegetable components it’s better to feed molly with flakes or you can give it pieces of boiled cucumbers, squash, lettuce leaves.

Live food you may add to the diet is bloodworm, tubifex, brine shrimp. In general, feeding isn’t complicated, the main thing is to remember that vegetable component is very important in the diet.


It is quite simple to see between the male and female. Females are larger and they have large rounded abdomen.

The most certain difference you can use to identify the sex is the shape of anal fin – the male has its anal fin modified into a tube (gonopodium) and females anal fin is of triangle shape.


Molly is a live-breeder just like guppy, which means that the juveniles are spawned into the tank being completely formed and they skip the egg stage. You don’t need any special conditions for breeding. It’s quite enough to keep males and females together in a common tank.

Provided with comfortable tank conditions fish spawns every month. When mating the male uses its gonopodium which has a tube to transfer ejaculate to the male and a kind of hook to stick closer to the female when fertilizing her, which increases chances of successful fertilization.

The males become reproductive at about the age of 12 month, and females when being 6 month old.

The female carries eggs for about 35 – 45 days. It’s difficult to see if the female is pregnant, especially in case of black molly.

As for the pregnancy signs – they are enlarged abdomen and the female starts to look for a shelter in some dark corners of the tank. It’s desirable to put such a female into a separate spawning tank where water parameters will be the same as that of the common tank.

Usually spawning occurs in the early morning. Large sized female may spawn up to hundred juveniles at once.

The number depends on female age and size. Juveniles are born rather large, they don’t get up from the tank bottom right away and stay there or on the leaves of tank plants for a while.

Juveniles are very sensitive to tank water contamination, that’s why it’s recommended to renew water in the tank with juveniles more often than usually.

To grow fast the juveniles need to have a diversified diet. They are fed with microworms, milled high quality artificial food or special food for juveniles.