What tank size does a Molly Fish need?
Mollies are social livebearers that do best in groups of 5 or more fish in a 20-gallon or larger tank. They are active swimmers and grow to 3–5 inches, so they need more room than their common pairing with guppies might suggest.
The minimum tank for a small group of 5 common mollies (Poecilia sphenops) is 20 gallons, with 30 gallons being more comfortable and easier to maintain. Sailfin mollies (Poecilia latipinna or velifera) are larger — up to 5–6 inches — and need at least 30 gallons.
Mollies prefer slightly hard, alkaline water (pH 7.5–8.2) and some aquarists add a small amount of aquarium salt to replicate their natural brackish-water habitats. This tolerance for salt is unique among common livebearers and can help treat some common molly diseases.
Like guppies, mollies are livebearers and will reproduce in a mixed-sex tank. Keep 2–3 females per male to distribute mating pressure and reduce harassment of females. A separate breeding/fry tank is useful if you plan to raise the young.
Frequently asked questions
What size tank do mollies need?
Mollies need a minimum of 20 gallons for a small group of 5. A 30-gallon tank is the recommended size for a comfortable community of mollies. Sailfin mollies, which grow to 5–6 inches, need 30 gallons minimum. Mollies are active fish and need more swimming room than their small size implies.
Can mollies live in a 10-gallon tank?
A 10-gallon tank is too small for a proper molly group. Mollies need at least 5 fish for stable social dynamics and grow to 3–5 inches as adults. Five mollies in a 10-gallon tank means only 2 gallons per fish — too cramped for their swimming style and waste output. A 20-gallon minimum is required.
How many mollies can I keep in a 20-gallon tank?
A 20-gallon tank can comfortably hold 4–6 common mollies (Poecilia sphenops). At 4 gallons per molly, a 20-gallon tank supports 5 fish. This assumes a properly cycled tank with regular water changes. If you are keeping a mixed-sex group, plan for fry — mollies breed readily in any aquarium.
Do mollies need salt in the water?
Mollies are naturally found in brackish coastal waters and tolerate salt well, which is unusual among common freshwater livebearers. Adding 1 tablespoon of aquarium salt per 5 gallons can reduce disease susceptibility and stress for mollies. However, salt is not required for mollies in a properly maintained freshwater tank — it is an option, not a necessity.
How many gallons do sailfin mollies need?
Sailfin mollies (Poecilia latipinna or velifera) are larger than common mollies, reaching 5–6 inches in length. They need a minimum of 30 gallons for a small group. A 40-gallon tank is more appropriate to accommodate their size and activity level. The standard 4-gallons-per-molly rule still applies, but start from a higher base tank size.
More Molly Fish resources