See title
Prerequisites:
1. Have enough weight in your inventory to enter "overweight" status
2. Have a fish tank with enough space
Scenario 1
3. Catch and auto-loot any fish
Fish will land in regular inventory and is unable to move to fish tank
Scenario 2
3. Have any fish inside your inventory
4. Open fish tank UI
5. RMB on fish in inventory
Fish won't go into fish tank and the message "Too heavy." appears on screen.

By design. It's in the description of the tank.
Bug Reports
Use this forum section to report bugs during your adventures.