Gph ? No idea prolly close to the max allowed for the 1800 model. But my pump runs like 2600gph@4ft of head, as it backed to about half throttle. I think is what a lot of people do is they don't have strong enough pumps and they gets air pocket in the top of the u-tube and when hydro goes out the pump isn't strong enough to push the water and force the air out. My pump will push a huge air pocket through, I've done multpile test with different size of air pockets and my pump will basically push enough water through to force the air pocket out. Very unlikely your u-tube would even get clogges unless you have small fish that would get in there, and also the only thing I dislike about the overflow box is the lack of amount you can adjust the height for inside the tank with the black box. Just wish I could move down a few more inchs then it moves now.
