if you did (Ie.. eheim,maxi-jets) you might try a strainer on the intake with a cover(like the use on phos reactor set ups & check your skimmer to see where its returning the skimmer water
Its not the powerheads the bubbles are going to the powerheads with the the skimmer off.............
I dont know where the bubbles are coming from? I tried to put a piece of foam to diffuse the bubbles and still no luck.......It is like a fine mist but it definitely affects your view.
Try cutting the foam block in the first overflow in half. Just use the half piece and keep it towards the bottom. Also try adjustint the water intake screen to a lower level