After playing the game as normal, exit out like you typically would. Now you can set your PlayStation 3 console's clock forward a few years (note that doing so may affect other games and settings, so be careful), then start playing the game again to find that you've gained a variety of fish. You can repeat this trick as often as you like. Some fish will grow older and larger as you repeat the trick, plus you should gain new varieties. Then you can return your clock to its normal time when you're done.