So that I understand what I am seeing, the siphon ( the black PVC
piping with the one way valve attached to it) draws water from the tank directly into the sump below?
Is there an overflow box that is fed by the siphon and drains into the sump?
What happens when you shut off the return pump? Does the siphon continue to draw water from the tank and fills up the sump?
Sound like the siphon cannot keep up with the output of the return pump.
Sorry so many questions...I don't see an overflow box so I am confused on how this is working.