It always depends on the fish species you want to keep. Generally, you can put up to 30 fish in a 30 gallon tank, but you should keep in mind that the more fish you have, the more bio-load your filter will need to be able to handle. That's why we recommend using a powerful filter for your 30 gallon fish tank. The only filter that can handle such a high volume of water is the Fluval U2 Underwater Filter.