How Many Fish Can You Put in a Tank? A Simple Stocking Guide

Worried about overstocking? Here is a clear, modern way to work out how many fish your aquarium can safely hold - without the dodgy old rules.

Forget the 'one inch per gallon' rule

You may have heard 'one inch of fish per gallon' (or one cm per litre). It is outdated and unreliable because it ignores how much waste a fish makes, how big it grows and how much swimming room it needs. A single large messy fish can pollute more than a dozen tiny ones of the same total length.

A better, safer approach

Stock by these four questions instead: How big does the fish get as an adult (not the size in the shop)? How much waste does it make? How much swimming space and territory does it need? And do its water needs match your other fish? When in doubt, understock - it is the easiest way to keep water clean and fish healthy.

Rough starting guide

As a conservative guide for small community fish, aim for around 1 cm of adult fish length per 2 litres of actual water, then reduce it for large-bodied or messy fish and increase it only with strong filtration and live plants.

Our fish stocking calculator does this maths for you, and the fish growth estimator helps you plan around adult size.

Golden rules

Add fish slowly over several weeks so your filter bacteria can keep up; never add a whole tank's worth at once. Keep schooling fish in groups of six or more. And always research a fish's adult size before you buy - many 'cute little' fish in the shop grow far too big for a home tank.

Frequently Asked Questions