You can't do that. It's a penguin power head. It's build just like a maxijet, and is set up correctly. the water rushing through the tube should pull air through the airline as it goes by. It's not a good skimmer. it's a seaclone or similar. Not going to do much good at all, but he's got it right from what I can see.
Maybe the skimmer is supposed to be in deeper water. It might make it easier for the pump to push water into the skimmer, thus creating less back pressure and allowing air in......just a guess...