Black Molly

These fish are one of the coolest to watch swim! It’s also quite obvious where this name comes from. Black molly fish are black, however, occasionally, you can also find a black molly with a yellow stripe down their body.


The black molly fish originated from ancestral species living in the coastal waters of Central and Southern America and Mexico. The species was bred in captivity from these natural ancestors by using selective breeding to choose specific, favorable traits.


Black mollies are one of the smaller options when looking for a new molly fish. Males can grow up to three inches long, and females may grow slightly longer than that. Like most fish, the lifespan of these creatures will depend on how well they are taken care of. So, how do you take proper care of a molly fish?

Gender Differences


The black fin molly has noticeable differences between the male and female individuals. The male, for example, usually has a much larger and more distinguishable sail fin; they are also usually larger than the females. Both genders of fish tend to have normal-shaped anal fins, though.

However, female fish are recognised on different factors. Not only are they smaller than their male counterparts, they also have a smaller sail fin. Furthermore, when pregnant – which in nature, the females would remain pregnant most of the time – the female’s stomach noticeably swells and develops a dark spot, called the gravid spot, at the base of the anal fin. This gravid spot is actually comparable to the womb in mammals, however, it serves a slightly different purpose in that it does not directly feed the young.

Care / Tank Conditions

In order to ensure a peaceful environment for your fish, be sure to not put too many male black molly fish in the same tank. Doing so can cause aggression, and female black molly fish in the middle of an excessive amount of males may be harassed too. As you can imagine, these are not the conditions you want to live in, nor are they the conditions your fish should live in. The ideal ratio of male and female black molly fish is 1:3. This should help avoid aggression in the fish tank. If your female fish are being harassed, get more of them in order to help divide the male attention between all of them. This will also help avoid having your female molly fish from dying because they weren’t able to eat since they were stressed.

Despite this breed having the ability to survive in a variety of water conditions, the ideal water type is freshwater. Since these fish also love space, it’s strongly recommended to have a large aquarium for them to be able to relax and swim around freely. Until a certain point, the more water in the tank, the better. However, you certainly want to ensure that your black molly fish have at least 20 gallons of water, anything up to 50 gallons is perfect too.

When it comes to finding the perfect water pH and temperature, you will want to aim for a temperature of around 75 degrees Fahrenheit, and around 7.9 or 8.0 on the pH spectrum.

Last but not least, have lots of plants in the fish tank! Black molly fish love plants and having lots of them will not only help your fish feel comfortable but will also help with reproduction since they will have somewhere to hide. Also, be sure to filter the aquarium water frequently.

What do black molly fish eat?


Picking out food is a pretty easy task. Most fish food works for black molly fish. These fish are algae-eaters meaning that if your aquarium is prone to algae breakouts, your black molly fish will certainly help keep it under control. To ensure your mollies don’t go hungry while your tank is algae-free, algae wafers are a cheap and nutritious option.

It is also great to feed your black molly fish small pieces of vegetables. The best vegetables to feed them are zucchinis and cucumbers. Feeding your fish these types of foods once or twice per week is sufficient.

Breeding Black Molly

Assuming that you are respecting the encouraged ratio of three females to one male, the breeding will happen on its own, in most cases. Once you have ensured that your tank is properly set up with plenty of plants and hiding spots, it is safe to expect some new black molly fish babies within the next few months!

Suitable Tank Mates for Black Molly

As you may know, not all fish breeds can be put in the same tank as other fish breed. Before buying a black molly fish, it is important to know which fish breeds they can be put in the same tank as. Guppies, Danios, Tetras, other Mollies, Swordtails, Gouramis, Platies and even female Betta fish are all fine to place in the same tank as your black molly fish.