I can't say that I for sure know the answer, but I just posted something very similar last night in the Disease/Sickness and Medication section. You may want to check out that post and see if it's the same thing. I think there are a few possibilities, but with my fish, I think it may be just undigested food due to overfeeding. I do know that a salt dip will relieve constipation, so perhaps you "unclogged" him.