Where to Put Fish in Super Paper Mario (Wii)? You Don't – It's a Fishing Contest Minigame in Chapter 3-2

You don't "place" fish anywhere. You're thinking of the Fishing Contest minigame (Chapter 3-2, The Bitlands):

1. Talk to the fisherman → enter contest.
2. Wii Remote: cast (flick), reel (rotate), pull (jerk back).
3. Catch as many as possible in time limit.
4. Judge awards rank + coins/items based on size/rarity.

Fish are not stored, displayed, or used elsewhere. It's a self-contained minigame for rewards.