Just a thought, but you might want to invest in the "tube sponge" for your skimmer. The individual sponge rings seem to come off over time and the tube sponge helps cut the airbubbles when the water exits the skimmer.
A tube sponge is a sponge that is a long cylinder with the center cut out and it goes over the return pipe. That way when the water flows down it cuts the bubbles and also it stays in tact as it is one piece. My LFS gave me one with my skimmer and said to use it vs. the rings. Probably just a personal preference thing...but the sponge works for me.