Drag, oh and by the way those of you useing the 1 inch overflows.....I had a tank in the garage overflow 15 gallons of water onto the floor because a turbo snail crawled over the overflow box, and up into the tube which restricted the flow and caused the flood. Remeber if you are useing the 1 inch to put in at least 2 and always watch for snails and such things that may constrict the flow. That is why I like the 1.5-2 inch bulkheads.