Go to Walter's house and become friends. He will then try to challenge you to see who has the largest trout from the lake. If you do this, Walter will hand over 200 Simoleons.

Kia will also give you 200 Simoleons if you give her a catfish.

Earn money by selling fish. Go to the Market and select "Shop" in the menu. You can then use the plus and minus signs to sell your stock of fish. Vegetables can also be sold for Simoleons.