If you are just starting, you must grow and sell fish that matures fast. These are the Mini Dart Goby, Red Spot Cardinal and the Sardine fish. You can grow them for about 5 minutes, 15 minutes and 3 minutes. You can earn coins fast and get experience points fast than raising a fish that would take you a day before you can sell them.
For more infomation on fish harvest times see our fish guide.