[header] How to choose fish for a freshwater aquarium [title] Choose fish that will fit in your aquarium. [step] If you already have an aquarium, you can use its size to help inform your decision regarding which fish to purchase. Do a little online research-or talk to knowledgeable sales staff at a pet store-to see how large specific species of fish will grow to be. 
[substeps] Most of the breeds of freshwater fish that you find at a pet store will fit in a 15-gallon tank. Although juvenile fish you see in pet stores may be small, as they grow, many species will become quite large.