Are you asking about the growth below his belly? It might be an anal/vent prolapse and it is suggested to use Epsom salt (magnesium sulfate) to help it reduce in size.
The recipe is 1 tablespoon plain, unscented Epsom salt (magnesium sulfate) per 5 gallons of aquarium water, dissolve the Epsom salt thoroughly in the water and add it slowly to the aquarium over the course of a few hours. If you have a large aquarium, you could move the fish to a smaller tank or use a plastic tote large enough to comfortably contain the fish AND that you can add a mature filter to.
DO NOT use aquarium salt, Epsom salt can be found in most drug stores or convenience stores in the first aid aisle.
Is this fish new to you or have you had it for a while? What are your test results since you posted you do test your aquarium parameters.
