How to choose fish for your tank

Updated 2026-06-146 min read

Choosing fish is where a tank gains its personality, and it is also where many beginners go wrong by buying on looks alone. The right stocking comes down to matching species to your water type, tank size, and to each other.

This guide walks through the checks that prevent the common disasters — aggression, overstocking, and incompatible water needs — so your community settles in peacefully.

Steps

Match fish to your water type
Decide freshwater, saltwater, or brackish first, then only consider species suited to it. Within freshwater, also match temperature and hardness preferences.
Respect adult size and tank size
Research the full-grown size, not the size in the shop. A fish sold at 3 cm may reach 30 cm; stock for the adult or you will be rehoming it.
Ignore the old “one inch of fish per gallon” rule — it overstocks tanks. Use adult size, swimming style, and waste output instead.
Check temperament and compatibility
Pair peaceful with peaceful. Avoid mixing fin-nippers with long-finned fish or aggressive species with timid ones. Verify every pairing before you buy.
Keep schooling fish in groups
Schooling species like tetras, rasboras, and corydoras are stressed and prone to nipping when kept in twos or threes. Keep them in groups of six or more.
Account for bioload
More fish and bigger fish mean more waste, which your filter and water changes must handle. Stock gradually and let your readings confirm the tank keeps up.
Inhabitants
Neon tetra ×8
Otocinclus ×4
Amano shrimp ×6
Add new fish slowly
Introduce a few at a time over weeks, not all at once. This lets the filter bacteria scale up and lets you spot problems before they spread.