Betta has an air pocket IN his fin?

Question
We have a male betta.  This afternoon I noticed that his left bottom fin was not closing right. I got out a mirror so he would puff up and I could see him better.  It turns out there's a pocket of air *IN* the fin itself and that's pulling it up so he can't put it down properly.  Have you ever heard of this happening and is there anything I can do for him? Is it hurting him?

We took him out two nights ago and cleaned his tank 'cause it was getting a bit filthy. Could the air have somehow gotten in there then (when we had him in the net briefly)?

He's the only fish in his bowl. It's a 1 gallon with a filter and gets cleaned once a week--50% water change and scrub for the walls.

Help?

Answer
This doesn't sound familiar to me. Unless if you are mistaking some sort of disease for an air pocket, I'm not sure there's much you can do. Do you have a bubbler in his bowl? If so, you should remove it. Bettas do not live in well oxygenated water, and it may be harmful to them to have something that is oxygenating the water. Make sure your ammonia and nitrite levels are at 0. If not, you'll need to start doing more water changes.