[Dentoid]

I was just guessing. Bristle worm setae can be poisonous and irritating. If he did try, or succeed at eating a bristle worm, it's plausible that the bristles are lodged in his gills causing difficulty in breathing. How to treat that is beyond me. Perhaps something that will increase production of the slime coat. I don't know of any NSAIDs for aquarium use.

First and foremost is getting a positive ID before treating.