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.
If you put fast fish in your tank you need to wath carefully. If you come back too late they will beb dead! It is harder than you think not to get distracted. It happend to me three times in a row with Gobies. Each time I tols myself "This time I am going to come back in time" and then I would get distracted. When I remembered about 10 minutes later there was no time to save them.