Try adding Epsom Salt at one tablespoon per gallon. Mix it up in a cup of hot water, then allow it to cool to room temp. Add dechlorinator, and then add it to the tank. You should add enough to "treat" half of the tank's volume tonight, and then half tomorrow. If it's going to work, it'll work within three days. This will relive constipation and aid in the reduction of swelling.
Are you feeding wet-frozen foods and bits of vegetable matter, like peas? These don't have to be the only thing fed; it's find to feed dried foods as long as you mix in some of these to aid in digestion and reduce chances of constipation.