Fun Lake
To get to Level 5, simply catch at least one of every type of fish including the guardian. At the end of the credits it will say Fun Lake unlocked. Then, you will start off in the first level.
Buy supplies if needed because there is nothing but an inn in level 5. Finally, go to the inn in level 1 and stay. Choose yes to go there. In level 5, there is a small lake that contains at least one of every fish.


Get rich quick
If you want lots of money, get a fly rod and go to the Stream. An Uncle will ask you for Ayu, and all you have to do is keep on catching them. If you run out of fish there, on the opposite side, there are more Ayu. Also, every time you go into a building, the Ayu will come back.


Music cheat
To play music while fishing pull in your line, but don't cast yet, push A and go to the tools menu. Once there go to Memo, Fishing record of how many and what kinds of fish you caught, once you are in your Memo thing. Exit and there will be music