If its just swimming up and down and back and forth in the currunt outside the tube then leave it as is, its normal. If the fish is attempting to go into the tube then you should put a bit screen (a knee high panty hose might work, i use them on intake tubes when i have tiny fry) over the end till the fish is too big to fit inside. Dont want it to get stuck and clog the tube making a mess in the house etc... I would be more concerned if it was trying to get into a inake tube...