The problem is that the overflow back to the tank is encorporating air into the water as it returns to the main tank. You have to allow that air to escape somehow. After the overflow comes out of the fuge you could add a jog up to the level of the top of the fuge, over a couple of inches, and then back down to the tank. Just instead of 3/4 inch 90's at the top of the jogs, put 3/4 inch Tee's with the top open. That should allow the bubbles to gas off.