reverse siphon

After some help please!!! Just set up a redsea300xl and I get the water level to stay around 60mm from the overflow and it stays constant until the backbox will drain rapidly, causing the pumps to push out oxygen then will fill back up again, hover around that level until it happens all over...

Hey, trying to find a check valve, two actually, to run in my return lines as a reverse check valve Melevs style that isnt going to break the bank. Does anyone have any suggestions? My return lines are 1". I think anything 1/2 or greater is enough air to break the siphon. Link to the idea: