The only limiting factor in choosing a fish for your Water Garden is that it's a 3 gallon tank. We recommend 1 Betta fish but if you prefer to have a few fish we suggest keeping 2-3 Zebra Danios or Male Fancy Guppies as an alternate pet choice. We've had folks put snails, shrimp, crabs, and a lot of other fish in there. Based on the average 1" of fish per gallon rule, you'll want to put only small fish in there and if they grow (like Goldfish), transfer them out to a larger tank. Also, the AquaFarm provides very little additional oxygen to the tank. If you have a different species of fish that require oxygenated water, like Goldfish, we recommend contacting your local pet store about adding an air stone or water plant to your tank.