What kind of skimmer is it? How long have you been runnig it? What other kind of mechanical filtration do you have? How old is your tank?
It is somewhat normal for a hang-on model to have some bubbles in the overflow. How many depends on the amount of DOC
you have built up in you tank. Eventually the amount of bubbles will decrease to a manageble minimum. How long this takes depends on the quality of the skimmer you bought, how often and what you feed, and the size of the tank.
All things said, the presense of the bubbles means that the skimmer is doing its' job. What you are seeing is that the more DOC
you have in your tank, the harder the skimmer has to work and more of the bubbles escape back into the tank. This will get less and less as DOC
are removed. HTH