Depending on your plumbing, it may just be the SWCG creating the bubbles - I had a pool last year that had huge bubbles that were coming out of the return closest to the pump - apparently there is a 90 elbow that will collect the minute hydrogen bubbles that are a byproduct of using a SWCG until they get large enough to be forced out into the pool, this may be what you are experiencing.

Any air in the system would have to come from the influent side (before the pump) and you would see it in the pump basket leaks after the pump would leak water, not draw air into the system.

Glad you were able to figure out the DE - I've said a few times that too much or too little will negatively effect the filter's performance