Three to five years, with conditions attached
A molly kept well lives 3 to 5 years. That places it in the middle of the aquarium lifespan range — longer than its guppy cousins, shorter than the small tetras — but the spread within that range is unusually wide, because mollies are unusually sensitive to being kept in the wrong kind of water. A molly in conditions that suit it is a bold, active, constantly grazing fish; the same fish in unsuitable water fades by slow degrees over months, and keepers often blame the species rather than the mismatch.
Bred for looks first, hardiness second
Almost every molly in the trade is a hybrid, produced by crossing wild species such as the sailfin and shortfin mollies and then selecting for black, gold, dalmatian and balloon body forms. That history explains both their variety and their fragility. Some lines, balloon mollies especially, carry a deliberately deformed spine that burdens swimming and digestion for the fish's whole life. Choosing a natural-bodied molly from a healthy, active shop tank is a real lifespan decision; the sturdiest-looking fish in the group, already at a good size, will almost always outlive the smallest and most extreme-shaped one.
Hard water is their home water
Wild mollies live in hard, alkaline coastal waters, some populations in water that is frankly brackish, and their bodies are calibrated for mineral-rich water. In soft, acidic water they lose condition insidiously: the shivering, clamped-fin posture keepers call the shimmies, susceptibility to fungus and fin rot, and a general failure to thrive. Long-lived mollies are kept in hard alkaline water — often helped by crushed coral or aragonite in the filter or substrate where tap water is very soft — at a warm 75 to 82°F. Many also relish a partly brackish setup, though a stable full-freshwater hard tank beats an unstable brackish one every time.
Space, greens and the breeding balance
Mollies are grazers built around a largely plant-based gut, and a long-lived one gets algae to pick at plus regular vegetable matter — blanched spinach, peas or courgette — alongside its staple food. They are also livebearers, and the social maths matters: a single female chased by several males spends her life stressed and pregnant, while two or three females per male spreads the attention and lets each female recover between batches of fry. Give them swimming room, since the sailfin males in particular grow into substantial four-inch fish. The warning signs that a molly's environment is failing it are the shimmies, white-edged eroding fins, and a fish that stops grazing and hangs at the surface — check hardness and temperature first, because for this species the water profile is usually the story.
KinsLe Family Sharing, for this tank, means an owner opening their profile to parents or other family members that owner names; the mineral habit itself stays protected because the household checks hardness before every refill.