hmmm how do you know you have micro bubbles? I don't think you could see them if they are micro bubbles, maybe you have Macro bubbles
seriously though how did you test all your piping? and did you rest it from the pump casing all the way to under the water line of the tank?
If you think it is carry over through the sump you could try putting a baffle in to cover the pump suction. Oh how does the water travel through the last baffle be for the pump in your sump?
Steve
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