So the air bubbles are coming out of the sand? How deep is your sand bed?
It usually means that the bacteria in the sand are participating in the process of denitrification (The process of neutralizing nitrates). It should be harmless, beneficial, and an indication that your sand bed is effectively acting as a nitrate filter.
Oh, and you may want to wait on the chaeto so that it does not have to go through the cycling process. Also, until the cycle finishes, there will be no nitrates and other things for it to thrive on. Hope this helps,
Mike15.