Molly Fish Care Guide: Tank Size, Water & Tankmates
The molly (Poecilia species) is a hardy, friendly livebearer that has been a community-tank favorite for generations. They come in a wonderful range of colors and fin types, from sleek black mollies to flashy sailfins, and they breed readily, which makes them a great pick for beginners. At Tropical Treasures, we keep a rotating selection of mollies in store, including the Black Lyretail Molly, the Marble Dalmatian Molly, and the Assorted Sailfin Molly. This guide covers everything you need to keep mollies healthy and thriving.
Meet the Molly
Mollies are livebearers native to the Americas, where they live in fresh, brackish, and even coastal waters. That adaptability is part of why they are so hardy. Common types include the short-finned molly, the lyretail, and the tall-finned sailfin molly, which can reach 4 to 5 inches. They are close relatives of platies, Endlers, and swordtails, and mix well with all of them.
Tank Size and Setup
Mollies are active swimmers that grow larger than many other livebearers, so give them room. A small group does best in at least a 20 gallon tank, with more space for sailfin varieties. Provide steady filtration, open swimming areas, and some live plants for cover and grazing. A secure lid and a well-cycled tank go a long way toward keeping mollies happy.
Water Parameters and Heating
Mollies prefer warm water of 75–82°F, a pH of 7.5–8.5, and harder, more alkaline water than most tropical fish. They are sensitive to poor water quality, so a reliable aquarium heater and good maintenance matter. Always use a quality water conditioner on tap water, and keep up with regular maintenance and partial water changes to keep nitrates in check.
Feeding Your Molly
Mollies are omnivores with a strong appetite for vegetable matter; in the wild they graze constantly on algae. A quality flake or pellet fish food with some plant content makes a great staple, supplemented with blanched vegetables, spirulina, and the occasional protein treat like brine shrimp. Feed small amounts once or twice a day that they can finish in a minute or two.
Tankmates and Temperament
Mollies are peaceful and well suited to a community tank. Good companions include platies, swordtails, guppies, tetras, corydoras, and other calm species that enjoy similar water. Avoid aggressive or fin-nipping fish. Males can be a little pushy with females, so keeping more females than males (about two to three females per male) helps spread out attention and reduces stress.
Breeding Mollies
Like other livebearers, mollies breed easily and give birth to free-swimming fry rather than laying eggs. A female can produce 20 to 60 fry roughly once a month, and adults will eat fry if given the chance. Provide dense plants or a separate grow-out space to help fry survive, and feed them finely crushed food or baby brine shrimp. Be ready to manage the growing population if you keep both sexes.
Is the Molly Right for You?
If you want a colorful, active, easygoing fish that brings movement to a community tank, the molly is an excellent choice, especially for newer hobbyists. Just give them room, keep their water clean and a touch harder, and plan ahead for fry. New to the hobby and unsure where to buy? Our guide on buying fish in store vs online explains why healthy, locally acclimated fish make all the difference.
Frequently Asked Questions
Do mollies need salt in their water? Not necessarily. While mollies tolerate and even enjoy slightly brackish water, they do well in clean, hard, alkaline freshwater. Adding salt is optional and not required for most community setups.
Do mollies need a heater? Yes. Mollies are tropical fish and need a stable heater keeping the tank in the upper 70s°F for good health and color.
How many mollies should I keep together? Keep mollies in groups of at least four to six, with more females than males. They are social fish that feel most secure in a small group.