Also, a closed loop is for circulation only. It is not a filtration method. That is the sump and other equipments job.Hi Shep - Not a dumb question! A closed loop is just a setup where you are taking water from the tank and pumping it right back into the tank, as opposed to e.g. letting the water overflow down into a sump and pumping it back up. The plumbing going to and from the closed loop pump can either use bulkheads in holes drilled in the tank walls, or it can be plumbed up and over the edge of the tank. In general the pump itself sits at the same height as the tank, so there is no head pressure and you do not need a pressure-rated pump to run one.