You can also do a false bottom. The one i have is working sweet. its in a 18x18x24 zoomed terrarium. You save some money this way on hydro balls. They false button is made up of plastic egg crate ,1 3/4 PVC pipe cut in 2 inch pieces and have a hole on the bottom so the water circulates. If you want more info and the original idea search thru the forum. Donlisk made i think a tread on this. Its in the DIY or the Vivarium tank section. It has a pump on the bottom with the hose sticking thru the substrate.All i do is connect the hose with a longer hose and I'm ready to pump.