Mollies sit at the warm end of the range
Of the common livebearers, mollies ask for the most heat. Wild mollies live in warm coastal streams, estuaries and even shoreline pools, and the aquarium strains — black, sailfin, balloon and lyretail — keep best around 24 to 28 degrees, warmer than guppies or platies in the same shop row. A molly held in the low twenties does not adapt so much as endure: it shimmies in place, grows a dull, dusty look to the skin, and becomes the first fish in the tank to break out in spots or fungus when anything else slips. Keepers who conclude mollies are delicate usually discover, thermometer in hand, that the fish were simply cold — warmth is not an optimisation for mollies, it is the baseline their health is built on from the first day.
Heat plus the water mollies actually want
A heater alone does not complete a molly setup, because the warmth travels with a water type: hard and alkaline, the liquid equivalent of their coastal homes. Soft, acidic water undermines them almost as reliably as cold does, so test both temperature and general hardness before blaming the fish. Size the tank generously — sailfin males are substantial, active fish — and place the heater where flow distributes warmth through the open middle water where mollies cruise, rather than letting a planted corner become a warm pocket and the far end a cool disappointment. Salt is the old debate: mollies tolerate and can enjoy a little, but a well-run hard-water freshwater tank at the right temperature keeps them thriving without it, and plants and tankmates often object to salt long before the mollies would ask for more.
Living with warm, hungry livebearers
At their proper temperature mollies are busy, algae-nibbling, fry-producing fish, and the tank routine has to match the pace. They appreciate vegetable matter in the diet — spirulina-based foods and the soft algae on tank surfaces — alongside ordinary flakes, and a warm molly eats and excretes accordingly, so filtration and partial water changes carry more load than the fish's peaceful manner suggests. Males display best warm: the sailfin's great dorsal goes up like a flag in a contented, well-heated male, while a cold male keeps it folded and drab. Fry arrive regularly in mixed-sex groups, so plan the population as you would for any livebearer — cover for the fry you want to keep, and a plan for the rest before the warm, healthy tank breeds itself past its own capacity within a few months.
The molly shimmy is a thermometer reading
Keepers have a name for the classic cold-molly picture: the shimmies, a rocking, on-the-spot swimming with clamped fins that is this fish's way of announcing the water is wrong — too cold, too soft, or both. Treat it as an instruction to measure, not to medicate: check temperature at molly level, test hardness, and correct whichever number is off, slowly. White dusting on the skin, fraying fins or several mollies shimmying together after you have fixed the warmth deserve prompt advice from an aquatic vet, since warm water also lets some infections move fast. Writing the temperature and hardness targets down in KinsLe inside the Pet Passport means the next person to troubleshoot this tank starts from the molly's actual recipe, not from a guess.