They are all useful for breeding other fish, but there are many ways to find all of the species. It can be difficult to decide which ones to keep.

Some species can only come from eggs, so those are the ones that you really want to keep. They are also useful breeding stock for creating other species. Highlight the following for spoiler information: Silky Shark and Twin-Fin Spotanus are two very good fish to keep, since they cannot be bred from other fishies.