Before you go and spend money try the RO tubing mod. It works great! and it will give you even finer bubbles. Just snip 3 inches of tubbing off your ro.
I tried the RO mod, and the bubbles are definitely finer and smaller... just scratching my head trying to figure out the physics behind it - just seems counter intuitive
I ended up reusing the airtube valve that came up the skimmer, and its giving me the same results at the RO mod (maybe tad smaller bubbles). I took it off as I thought more air was better, but never really paid attention to the bubble size until now...