Peas and daphnia, i thought you said the buldge was behind the head, anyway here is some information on consitpation, you will need to introduce some frozen foods in there diet, like bloodworms, daphnia which is good for digesting there food, brimestrimp etc, and veg like peas, broc, cabbage, cucumba etc.
Constipation- This is often caused by a diet without enough variety, or by feeding too many starchy type foods. Symptoms: If your fish is having trouble passing feces, or if they are forever trailing behind a fish, your fish may be suffering from constipation. Treatment: Feeding your fish a diet with more roughage and a greater variety sometimes helps! Feed live foods such as (wax worms, brine shrimp, tubiflex worms, etc.) peas, spinach. If you can get your fish to eat one grain of Epson salt it can also help aid with the constipation. Soaking dry fish food in some water taken from the tank will soften the food and help in the digestion process. (back to top)