That can range from overeating to internal problems like intestinal blockage, a type of dropsy or internal infection, to internal parasites. Can't say from the little information provided.
Monitor the fish and note whether it is decreasing or increasing in size. Provide a photo if you think it is getting larger. Prepare a hospital tank now, just in case it will be needed shortly.
If you need to get back to us with the matter getting worse, provide more information as well as photos:
How old is your tank? When did it originally cycle?
What is the size (dimensions and gallonage) of your aquarium? Does the gallonage include the volume from the sump and any refugium?
Do you use carbon, skimmer, mechanical or other chemical filtration?
What kind of substrate and particle size is in your aquarium? Deep bed or how thick?
List all specimens & sizes in the tank (fish, inverts, corals, clams, snails, crabs, shrimp, etc.).
Do you use a quarantine tank and procedure?
Foods you use and feeding schedules.
Do you use any vitamins? Fat additives? Any elemental or other additives? Please list all.
Chemistries – you gave none. Do you test for Phosphate, alkalinity, calcium and magnesium? Please give actual current numbers for everything. Have these numbers been changing lately?
Water parameters – please give actual numbers (pH and your pH range, salinity or sp. gr. & range, temperature range)
Do you see any of the following in your system: hair algae; micro algae; cyanobacteria growths (red slime algae); dinoflagellate (zooxanthellae) growths; brown algae; diatom growth; slimes; off-colored patches on rock or substrate that are not coralline; etc.?
Water changes (how much and how often).
What is your source water? (Tap water, RO water, DI water, RO/DI, distilled, etc.)
List what you added or taken out of your aquarium system (living, decorations, and equipment) during the past 6 weeks.
Maintenance schedule. What have you done lately?
These fish are omnivores. Has it been eating algae and other greens? Has it been eating a balanced nutritious diet?
