Candidate
Betta fish
Betta splendens
A single fish plan leaves room for plants, gentle filtration and an accessible surface.
- Adult
- 5–7 cm
- Minimum
- 20 L · 45 cm
- Group
- 1
Condition-led guide · reviewed 2026-07-31
Fish for a 40 litre aquarium
A short, welfare-first list for compact mature aquariums, where footprint and group size remove many shop favourites.
Selection rules
What made the shortlist
- Use at least the profile's minimum footprint, not volume alone.
- Choose one primary fish plan rather than combining every candidate.
- Match source-water hardness before selecting a species.
Real photographed species
Options worth researching
Build around one species, preserve open swimming space and cycle the filter fully before stocking.
Candidate
Endler's livebearer
Poecilia wingei
A male-only group can suit hard water without creating continuous fry.
- Adult
- 2–4 cm
- Minimum
- 35 L · 45 cm
- Group
- 6
Candidate
Chili rasbora
Boraras brigittae
A generous single-species shoal fits a mature, gently filtered planted setup.
- Adult
- 1.5–2 cm
- Minimum
- 35 L · 45 cm
- Group
- 10
Common mismatch
Fish to rule out for this brief
Forty litres does not automatically mean 40 cm long. Check internal dimensions and every candidate profile.
Think again
Common pleco
Hypostomus plecostomus
Its adult length and waste load require a vastly larger aquarium.
- Adult
- 30–50 cm
- Minimum
- 450 L · 180 cm
- Group
- 1
Think again
Clown loach
Chromobotia macracanthus
A social group needs a much longer, very large adult home.
- Adult
- 25–35 cm
- Minimum
- 450 L · 180 cm
- Group
- 5
Think again
Bala shark
Balantiocheilos melanopterus
This powerful schooling swimmer is wholly unsuitable for a nano footprint.
- Adult
- 25–35 cm
- Minimum
- 600 L · 200 cm
- Group
- 5