It is best to raise 3, 6 or 9 fish. 3 and 6 mean "three-three inexhaustible" and "six-six infinite", and 9 is the largest single digit, which has an enduring meaning. For example, a fish tank can raise 19 or 29 fish. You can also hang "Nine Fish Map" or "Twelve Fish Map", which means "more than enough for a long time" and "more than enough for a month".
If you want to know more, you can search Chihiro Traffic Forecast Center.