Instructions
Start the aquarium level and look around the tank. You can get a panoramic view by using the directional buttons. Examine the tank until you see a black fish with white polka dots.
Use the Wii Remote to aim your cursor at the fish. You'll see that it has small white polka dots along one side while the other side has regular polka dots and one larger white dot.
Press "A" while your cursor is on this larger white dot of the fish to collect the white balloon.