If the bucket is going to drain into the display tank then it will have to be higher than the display tank. The pump will have to overcome the weight of the water in the tube going up to the bucket from the tank (head pressure). Just measure how far above the tank the return fitting is and that will be your head pressure. If you seal everything water tight you could probably mount the bucket even to or lower than the tank but the chances of a leak and the resulting flood would (IMO) not be worth it.