I agree with @Guppysnail possible internal growth or egg bound heavy parasite load can also cause this type of bloating what I would suggest is trying Epsom salt baths 1 table spoon for 1 gallon for no more than 15 minutes for 5 days as Epsom salt acts as a muscle relaxant I would fast the fish for a couple days as well if your seeing no improvement after 5 days something else could be going on like a tumor or heavy parasite burden @Pizzafan