We have an in-ground pool that has a waterfall with a 1.5 HP pump. Recently, we noticed that when the waterfall is turned on, the regular pool pump is not sucking in water through the main drain and skimmers. But when we turn off the valve to the waterfall, the main drain and skimmers work great. Does anyone know what would cause the main drain and skimmers to not function correctly when the waterfall is on? I suppose an easy option would be to turn off the main pump while the waterfall is on so it doesn't burn up the pump, but I'd prefer to figure out why it's doing what it's doing.