I recently repaired the liner in my pondless waterfall system. I noticed with the pump off, both biofalls will empty back down into the main reservoir over a period of a day or so causing the reservoir to rise accordingly. I can fill both biofalls back up from the reservoir full again, so i know there is no leak and the system is closed. I replaced the check valve with a new one, and still the same situation. My question is: is when both biofalls empty into the reservoir and doesnt cause an overflow (system still closed), do I need to have a check valve anyway? Wont the pump adequately keep the water circulating in a continuous fashion? Or eventually would the biofalls empty enough so as to not overflow and i would lose the waterfall?