How Many Fish Can I Stock?
Working out how many fish you should have in a tank is very tricky as there are countless numbers of factors that can affect 'acceptable' stock levels. The two main considerations are the requirements of the fish species you are keeping, and the amount of waste your filter can cope with.
In theory, an aquarium can hold as many fish as the filter allows. By this we mean that the waste produced by the fish must not be too much for the filter to handle.
The 'old' stocking guideline, which is often repeated, is roughly about 1cm of fish per litre, where the length of fish is the fully grown body size (not including the tail). With modern filtration, correct feeding levels, and proper aquarium maintenance, an aquarium can cope with a lot more than this.
A better guide is to use a sliding scale depending on the quality and capability of your filtration as follows:
Internal filter 1cm per litre
Oversized Internal filter 1.05cm per litre
External filter 1.1cm per litre
Oversized external filter 1.15cm per litre
The above stocking level guides are only guides and a little common sense also needs to be used. The fish you keep may not like lots of company, or may produce lots of waste, or grow big, and stocking guides do not take these into account.
For a more detailed guide to stocking levels read our article on Stocking Levels