How big do mollies get?
Common mollies reach 8–10 cm (3–4 inches); sailfin mollies up to 13 cm (5 inches). Balloon varieties stay smaller but carry a shortened, curved spine — a body shape we do not recommend breeding or buying.
How long do mollies live?
3–5 years with hard, clean water and no constant breeding. Soft or acidic water, overbreeding and small tanks are the three things that shorten molly lifespans the most.
Molly tank size and water parameters
A group of mollies wants 75+ liters (20+ gallons) — they are active and messy. Temperature 24–27 °C, pH 7.5–8.5, hard water. A small dose of aquarium salt is a traditional, well-tolerated addition for mollies specifically; do not add salt to tanks with live plants or scaleless tank mates.
What do mollies eat?
Mostly plants and algae with a quality flake or pellet base — they are heavily herbivorous for livebearers. Offer blanched zucchini, peas and spinach regularly; the algae-grazing reputation is real, and a molly without greens is a molly with a compromised immune system.
Molly tank mates
Good: platies, guppies, swordtails, corydoras and other peaceful community fish. Avoid: fin-nippers such as tiger barbs, and any aggressive cichlid.
Why is my molly shimmying or lying at the bottom?
Shimmying (rocking side to side while otherwise active) is the classic molly response to water that is too soft, too acidic or too cold — fix the parameters first. Lying at the bottom after shipping or a water change is usually stress and passes; combined with clamped fins, hiding or color loss it points to illness. Persistent shimmying that does not resolve with corrected water deserves a vet or experienced fish keeper's look — it can also be a symptom of "molly disease", a progressive bacterial infection.
How do mollies give birth?
Mollies are livebearers: a single female delivers 20–60 fry every 4–6 weeks with no special trigger. Adults eat the fry unless the tank is heavily planted or the mother is moved to a breeder box — plan for babies before they arrive.
Beginner mistakes to avoid
- Keeping mollies in soft, acidic water because "they're community fish" — it is the single biggest cause of molly decline.
- One or two mollies in a 40-liter tank; they need room and a group to be healthy.
- Adding salt indiscriminately to a community tank with plants or scaleless fish.
- No plan for fry — within two months you can have more mollies than tank.