I don't think there's anything wrong you are doing, the air mixture valve u installed should help control the amount of bubble skimmer makes by reducing air amount. However it does look like there's a small crack on the side of the skimmer where the seam is that's leaking air bubbles into the tank. There should be no air bubbles leaking from side of skimmer.
here's someone else who experienced same thing as you but his didn't have a leak on side of the skimmer.