a closed loop takes in water , gravity fed from the tank directly into a pump that simply pushes it back in. alot of people have their tanks drilled for drain and return, but we only drilled mine for drain, so my return is a manifold ring on the top. this replaces the bulky powerheads in the tank. powerheads that have life spans, that generate heat, that can break down and burn up, that are not very powerful. i freed up 5 outlets and will be saving on evergy costs as well.