There's no way anyone can tell you exactly what size PH
or air pump you'll need. There is a way to figure that based on the size of the skimmer, but I'm afraid it's over my head. What I would do is get the best pump I could afford...preferably one that has a rheostat so you can adjust the output. The Luft pump comes to mind if they still make it. I'd probably go with either a Maxi Jet 900 or 1200 and plan on using a valve to control the flow. It really depends on what size the skimmer is.