Matching the Right Fish to Your Tank Size
One of the most common mistakes in the aquarium hobby is buying fish that grow too large for their tank. A two-inch fish at the pet shop can become a ten-centimetre nightmare in six months. Here’s how to match your fish to the right tank — and avoid the heartbreak of rehoming or losing them.
Small Tanks (Under 40 Litres)
Tanks under 40 litres are often marketed as “beginner setups,” but they’re actually harder to maintain than larger tanks because water parameters fluctuate rapidly. If you’re set on a small tank, stick to:
- Betta fish (one male) — the classic small-tank resident, though they prefer 20L+ minimum
- Neon tetras — a small school of 6-8 in a 30L+ tank works well
- Cherry shrimp — fascinating to watch and they breed readily
- Endler’s livebearers — colourful, active, and stay small
Avoid goldfish entirely in small tanks. Even “fancy” goldfish need 80+ litres because they produce heavy waste loads and grow to 15-20cm.
Medium Tanks (40-100 Litres)
This is the sweet spot for most hobbyists. The larger water volume is more forgiving, and you have real options:
- Guppies, platies, mollies — colourful livebearers that thrive in groups
- Corydoras catfish — adorable bottom-dwellers that clean up leftover food (keep in groups of 6+)
- Rasboras and dwarf gouramis — peaceful community fish with personality
- Neon tetras and cardinal tetras — a school of 12+ creates a stunning display
Large Tanks (100+ Litres)
Bigger tanks unlock the most rewarding aquarium keeping. With 100+ litres, you can keep:
- Angelfish — majestic centrepiece fish that need vertical space (50cm+ tall)
- Bigger cichlids — electric blue acaras, keyhole cichlids, and cockatoo cichlids
- Schooling fish in proper numbers — 20+ cardinals, rummynoses, or Congo tetras
- Plecos and larger catfish — bristlenose plecos stay manageable; avoid common plecos which grow to 30cm+
The Golden Rule
Research the adult size, not the shop size. Every fish species has a maximum adult size published in reliable databases. Use that number, not what you see in the tank at the store.
A general guideline: allow 1 litre of water per centimetre of adult fish size for community species. For messy fish like goldfish, double that. This isn’t a perfect formula, but it prevents the worst mismatches.
Don’t Fall for the “Grow Into It” Myth
There is no such thing as a fish “growing to fit the tank.” A fish kept in too-small quarters won’t stay small — it will be stunted, stressed, and prone to disease. Stunted growth means internal organs are compressed while the fish is still growing. It’s not cute; it’s cruel.
Buy the right size tank first, or buy fish that will stay small. Your wallet and your fish will both be better off.