When considering how many fish you will be able to keep in a 20-gallon freshwater tank, determine what number will allow you to maintain the best conditions for the health of your pets.
The Basic Rule
An often-used rule of thumb is you can keep 1 inch of fish per gallon of water. Several considerations will help you arrive at the true, correct number.
Before deciding on the number of fish, research the species you are interested in. You will need to know their size once fully grown and their usual temperament.
Under the Basic Rule
Stay under the basic rule if the fish you like will get large. Larger fish need more space per fish. They also produce more waste, which will affect water quality. Also, if you have selected species that tend to be territorial or like a lot of swimming space, stay under the basic rule.
Up to the Limit
Go up to the limit if the fish you are planning to get are small to medium in size and peaceful in disposition. Also, you can go to the limit if you will be keeping live plants.
Once you add gravel, decorations and plants, the tank actually holds a little less than 20 gallons. No matter how many fish you settle on, do not get them all at once. Bring home a few, observe them for a few days to a week to make sure that they are thriving, then add more.