How Many Mollys in a 55 Gallon Tank?
About 7–9 — based on adult size, bioload, and real water volume.
About 7–9 molly
Photo via Wikimedia Commons
In a 55-gallon tank you can comfortably keep about 7–9 molly — enough for a proper school, on their own with good filtration and regular water changes. Add fewer if you're mixing in other species.
How we get that number
An adult molly reaches about 4.0" and has a moderate bioload. Working from real water volume and waste output (not the old one-inch-per-gallon myth), a 55-gallon tank supports roughly 7–9 of them. Because molly are schooling fish, keep them in a group of at least 3 — never just one or two. They need 72–80°F water at pH 7–8.2.
Build your exact stocking with the calculator → Full molly care guide →
Gear for a 55-gallon tank
As an Amazon Associate we earn from qualifying purchases.
FAQ
How many molly can I keep in a 55 gallon tank?
About 7–9 molly, kept on their own with good filtration. Add fewer when combining with other species — use the stocking calculator to be sure.