When starting, do not be overly concerned about the types of fish that you can get. Simply be sure to have the maximum amount floating around and feed them until they become adults. This allows you to get more ingredients for the more difficult to obtain fish. Releasing a certain number of fish in each set earns you a set mastery, which gives you rewards such as premium tickets for the Hermit Crab. Use the basic breed, the first fish that is available. A lot of them are required to get the materials for the more difficult to obtain fish. Consider focusing on a set at a time.