For most tropical community fish, the safe answer is none: 20 litres is below the general tank-size guidance from the RSPCA in the UK and Australia. A narrow exception is one betta in a properly equipped, mature aquarium. That does not make a 20-liter tank suitable for a shoal or mixed community.
Why 20 litres is too small for most tropical fish
Tank size is not just a question of whether a fish physically fits. Fish need room to swim and, depending on the species, companions; a small volume also makes temperature and water quality less stable. A heater, filter and careful maintenance are essential, but they do not make an undersized aquarium suitable.
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Recommendations differ by organization and region. The RSPCA UK advises choosing an aquarium larger than 45 × 30 × 30 cm, or about 40 litres, and notes that larger tanks provide more stable temperature and water conditions. RSPCA Australia sets a 50-litre minimum for tropical fish. These are welfare recommendations, not a universal legal standard.
Sources: RSPCA UK, “Choosing an Aquarium for Pet Fish”; RSPCA Australia, “How should I care for my tropical fish?” (updated 1 May 2024).
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The limited exception: one betta
RSPCA Australia says 20 litres or more is ideal for one Siamese fighting fish (betta) to display normal activity. OATA also recommends at least 20 litres for one male betta and says a filter is essential. This is a species-specific recommendation for a single fish, not a general stocking allowance.
Do not use the betta exception to add a shoal or other tankmates. RSPCA Australia recommends at least 40 litres for a betta kept with a small community shoal.
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Sources: RSPCA Australia, “How should I care for my Siamese fighting fish?” (updated 1 May 2024); OATA, “How to look after Siamese fighting fish (Bettas)”.
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Why “just a few tiny fish” is not a good workaround
Many popular tropical fish are social shoalers. RSPCA UK identifies danios, tetras, rainbowfish, rasboras and many catfish as examples, and advises keeping many shoaling species in groups of six or more to help avoid stress. Keeping one or two small tetras in a 20-liter tank does not meet that social need, while a proper group would need more room.
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Other tropical species may fight with their own kind or with different species. A fish’s adult size alone cannot determine whether it belongs in a tank: behaviour, swimming needs, compatibility, water conditions and the aquarium’s shape also matter.
Source: RSPCA UK, “Keeping Pet Fish Together”.
What a 20-liter betta setup needs
Before choosing even the single-betta option, confirm that the aquarium can meet the fish’s needs as a complete setup:
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- ⚠️ IMPORTANT SAFETY REMINDER - Do not bury the heater in sand or substrate. Keep at least 2 inches of clearance from the water surface. Always ensure the heater is fully submerged before plugging it in. Unplug the heater and let it cool for 10–20 minutes before removing it from the tank for cleaning or water changes.
- Suitable filtration: OATA says a filter is always essential for bettas.
- Species-appropriate heat: Tropical fish need a heater set for the species. Use a thermometer to check the water temperature.
- A mature biological filter: Allow the tank to establish before adding fish. When stocking, add fish gradually so the filter bacteria can adjust to the increased waste and feeding.
- Water testing: Test ammonia and nitrite; OATA says both should be zero before continuing to add stock. Once a tank is established, SPCA New Zealand recommends monitoring water quality at least weekly.
- Suitable water and space: Check the betta’s water requirements, swimming space and compatibility. Do not assume that equipment or frequent testing cancels out the tank-size limits.
Sources: OATA, “How to set up and look after a freshwater tank / aquarium”; SPCA New Zealand, “Caring for fish”; RSPCA Australia, “How should I care for my tropical fish?” (updated 1 May 2024).
How to decide what to do with your tank
- If you have not bought fish: choose a larger aquarium before selecting tropical community fish. For a 20-liter tank, consider only the single-betta option if you can provide the full setup and care it needs.
- If you already have the tank: identify the exact species you are considering and ask a reputable fish-care specialist whether your tank’s dimensions and setup meet its needs before purchase.
- If the tank is already stocked: do not add more fish based on a fish-per-litre calculation. Check water quality and seek species-specific advice about a suitable long-term setup.
A nominal 20-liter capacity does not state how much water remains after substrate and decorations are added. Nor can a universal litres-per-fish rule account for adult size, behaviour, water parameters and compatibility. As OATA puts it, “It is not possible to say exactly how many fish your aquarium can hold.”
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