I have a spa and UG 20000 gallon pool and I have to keep the Spa drain valve closed all the time as if I don't the spa will drain down to the pool level when the pump shuts off. How do you keep the Spa water circulating besides opening the spa drain when the pump is running and then run out and shut the drain closed when the pump shuts off??? My spa gets green real fast unless I open the drain and brush the heck out of it and then close the drain. Is there a check valve that I could put into my system to stop the spa from draining to the pool level when the pump shuts off?