OK. So it's up and running with just fresh RO/DI water for now. I've never had a tank with drilled overflow/return before. I'm finding that every 20 seconds or so, the water in the overflow (corner overflow) compartment reaches a top point, then goes down about 3 inches with a gurgling/crashing sound that injects gazillions of bubbles into the water coming into the sump. I've tried dialing back the return pump, but it's not helping. Any advice on what's happenening and how to correct this?