The short answer
A 40 gallon (roughly 150 litre) tank is a genuinely versatile community size. You can comfortably keep two or three shoals of small fish plus a centrepiece — for example 10–12 tetras, 8 corydoras, and a pair of rams — as long as the tank is cycled and well filtered. That’s a guideline, not a hard cap: stock by the adult size, waste output (bioload) and temperament of each species, not by a number.
Why the old formulas don’t work
“One inch of fish per gallon” and similar shortcuts ignore everything that actually matters. A single messy fish like a common pleco produces more waste than a dozen tetras, and schooling fish need room to move rather than a fixed inch each. What really sets your limit is filtration capacity and how much waste your beneficial bacteria can process. Two things decide a healthy stock: strong biological filtration and consistent maintenance.
Sensible 40 gallon stocking ideas
- A community: 12 neon tetras, 8 corydoras, and a pair of German blue rams
- A livebearer tank: a group of guppies, platies and mollies (keep 2–3 females per male)
- A gourami setup: a pearl gourami centrepiece over a shoal of rasboras and a corydoras group
Remember that schooling fish need 6 or more of their own kind, and avoid mixing fish with clashing temperaments.
Before you stock
Cycle the tank first so the filter can handle the load — see how to cycle an aquarium. Match your filter to the volume via our aquarium filters hub, plan the mix with how many fish you can keep, and if you’re choosing the tank itself see the best large aquariums. To design the community properly, read how to plan a community tank.