I have a 26x26 cube rimless tank. It has an external overflow with 1&1/2 in bulkheads. I have a very simple overflow, as its just a little bit of 1" pipe with 90s on it. I am running an Iwaki 20rt, which pushes 492 gph. I am getting lots of bubbles in my sump. The good thing is that they aren't returning to my tank at all, but what can I do to alleviate the bubbles.