Why won't the water level in my DIY skimmer stay steady? It's driving me nuts! The body of the skimmer is about 4 feet high, with a 3 inch diameter. A small mag-drive pump moves water from the tank through a 3/4 inch hose into the skimmer near the top. The water siphons back into the tank through an output hose at the bottom. I adjust the pump by partly closing a ball valve in the upper hose until the inflow matches the outflow. It will hold for awhile, but after several hours (like overnight) the water level usually drops a few inches. Why won't it hold steady? Thanks for any help you can give.