1. Put your finger on the red control stick and move the fishing line so it's above the key in the fish tank.
2. The location changes each time so just make sure in your game your fishing line is above the key.
3. Use the red control stick to move the fishing line down and grab the key.
4. Again using the red control stick, pull the fishing line all the way up and out of the tank and the key will appear on the ground. Pick up the key and use it on the door.