The short answer
Mollies are larger, messier livebearers than guppies, reaching 8–10 cm, so they need more room than their small cousins. In a 75–100 litre tank keep a group of 4 to 6 mollies, and if you mix sexes use 2–3 females per male. As with all livebearers, they breed readily, so plan for fry and don’t stock to the brim.
Why mollies need space and the right ratio
A molly is a big-bodied, active fish with a substantial bioload — a few of them foul a small tank fast. That’s why a 20 or 40 litre nano is a poor fit; a 75 litre+ tank holds water quality steady and gives them swimming room. On top of size, males harass females persistently. Keeping more females than males spreads that pressure so no single female is chased to exhaustion. A lone male-and-female pair almost always overworks the female.
Sensible molly stocking
- A trio-plus: 1 male + 3 females in a 75 litre, or scale up in a larger tank
- Males only: 4–6 male mollies to skip the fry entirely
- Mollies with compatible livebearers such as platies and swordtails
- A cleanup crew of nerite snails
Give them slightly harder water and avoid overcrowding — mollies are sensitive to poor water quality.
Before you stock
Cycle the tank first — see how to cycle an aquarium. Read the full molly care guide, check compatibility with can guppies and mollies live together, and get the ratio right via the livebearer ratio guide. For a suitable tank, browse the aquariums hub.