You can try an Epsom salt bath once a day. Epsom salt (magnesium sulfate) works as a laxative and helps with fluid retention. It's available in any drugstore in the first aid section for about $1 for a box.
Mix 1 level teaspoon per gallon of water (use tank water) in a bucket and add him. Observe for 20 minutes then gently place him back in tank.
Are his scales lifted or rough looking? This will be more obvious if you look at him top down as they will appear to stick out. What do you normally feed him (specifically)? Do you have gravel or pebbles as a substrate? What do your parameters read? These questions are important to figuring out what is happening here and the best means for treatment.
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