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Best fish for a 40 litre / 11 gallon tank

20 species in the library list a minimum of 40L or less (about 11 US gallons). 115 species need more water than that. This page shortlists. It does not replace the tank builder or pair checker.

A 40L is the smallest tank where a proper nano setup works, but only one. A school of six chili rasboras lives here, or a colony of clown killis, or a cherry shrimp colony with a few otocinclus. The room for an algae crew is the same room for a centerpiece. You pick one role, then build to it.

Stocking philosophy

40L is one schooling species or one centerpiece, never both. Pick the role the tank plays before buying the fish.

40L mistakeBuying the fish at the length you see in the shop, not the adult length on its profile. The shop length is six months old at most.

Open tank builder (40L)·Pair checker·All fish

Good first pass

Peaceful, beginner or intermediate care, not predatory, and not a fin-nipper in a small footprint. Adult size is capped on very small hubs. Still open each profile.

Beginner-peaceful picks

Beginner care, peaceful, not a fin-nipper, not predatory. Tighter than the list above.

Stocking ideas for a 40L

Starting points in words, not drawings. Nothing here is a buy list. Use the tank builder and pair checker on the real mix.

  • One species in a 40L

    Amano Shrimp can fit this volume on paper. A mixed community usually cannot. Pick one species you actually want to watch, stock a proper group, and leave it alone until that group is settled. Check every new species in the pair tool, then load the real list into the tank builder before you buy. The juvenile in the shop bag is not the adult you will keep.

Caution (fits the litres, not a free community pass)

Fin nippers, semi-aggressive fish, advanced care, big groups, or species that may eat smaller tank mates. Can work in a mature, species-appropriate setup. Not a starter mix.

Specialist / high-load (even if min ≤ 40L)

Predatory, aggressive, advanced, large groups, or may-eat-small. Not a typical community-first stock.

Too big for a 40L

Most species sold as beginner fish (mollies, angelfish, fancy guppies in groups) do not live in 40L long-term. They survive while they are small, then outgrow the tank inside the first year. Plan the adult size first, the juvenile size in the shop second.

These species list a higher minimum volume. They belong on a larger hub, not in this tank class.

Common mistakes at this volume

  • Buying the fish at the length you see in the shop, not the adult length on its profile. The shop length is six months old at most.
  • Under-schooling tetras, rasboras, or barbs. The fish look sick during the first week, and the diagnosis is almost always group size, not disease.
  • Adding a centrepiece fish like a gourami or a small predator without checking the minimum-tank line on its profile. A 60L tank punishes one bad addition harder than a 200L does.
  • Stocking the small tank as if it were a big one because the fish look small in a bag. Bioload and oxygen headroom come from volume and surface area, not from how the fish look in your hand.
  • Mixing fin-nippers with slow or long-finned fish without checking the fin-nipper flags. Most personality clashes you read about online are layout problems with a named scapegoat.
Full suitable list (20) — same min-tank filter as above

Every freshwater species in the library with a min tank of 40L or less. Prefer the sections above for decisions.

Plan a 40L stock

Shortlist from this hub, then run the real mix through the builder before you buy.

Open pair checkerOpen this plan in builder

Equipment for a 40L

A stocked 40L community needs a filter turning over roughly 160 L/h and a heater in the 50-100W range. Undersized filtration is the most common cause of the ammonia spikes that kill week-one stock.

Filter size guide →Heater size guide →Tank setup checklist →

Related guides

Planted tank? Beginner plants · Low-light plants · No-CO2 plants

Other tank hubs

Categories in the suitable list

Common questions about a 40L tank

What fish can I keep in a 40 litre tank?
Any species with a minimum tank at or below 40 litres on its profile shows up on this page. The two short lists at the top (good first pass, beginner peaceful) are the right starting point for a community tank. Run any pairing through the pair checker before stocking, since this list filters by volume alone.
What is the most common mistake at 40 litres?
Buying the fish at the length you see in the shop, not the adult length on its profile. The shop length is six months old at most.
What else goes wrong at this tank size?
Under-schooling tetras, rasboras, or barbs. The fish look sick during the first week, and the diagnosis is almost always group size, not disease.
Should I use litres or the tank builder for a 40L stock?
Use this hub to shortlist, then load the real mix into the tank builder and open a pair check for every new species. Volume alone never proves a community works.