What kind of skimmer is it? Is it new? Or used? I gather it's a venturi of sorts if you don't already have an airstone. But food for thought, sometimes reduced bubble production can be indication of a partial blockage or a gunk buildup somewhere in the venturi or the airline. When I ran a venturi skimmer I found that I would get a calcium buildup at the air injection point in the venturi. Cleaning it wasn't good enough, I had to take something like a drill bit and bust up that calcium buildup (and/or soak in vinegar for a few hours to really get it out of there).
It could also be indication that the pump used to run the skimmer may not be strong enough.
Not that adding an airstone isn't a bad idea as a general mod of some kinds of skimmers, but, I just remember the days of looking at my venturi, thinking, "gee that bubble production doesn't seem to have a lot of oomph" but I found usually it was indication that it was time for some "skimmer detailing" so I thought I'd chime in my $0.02 from the peanut gallery.
