I agree with Yaksplat. The air is getting in their from another source or it was trapped in the when you put the PH
under water. Follow his advice and see if that helps.
I must have lucked out when I built my sump. I have no baffles at all down there and I get ZERO microbubbles returned to the tank. This is with a PH
down there that is attached to my CPR overflow who's only purpose it to pull air out of the overflow so that the siphon doesn't break. The PH
pulls that air right into the sump and it floats to the surface.