Sounds like he has swim bladder problems. OR a tumor and that can not be treated.. YOU can try sulfur meds AND I DO RECOMMEND A DIFFERENT TANK!!
I want you to know that all the meds I read on says more of a sulfur ingredient is best. but this is the only MED that came up on my search, but NOT the only med you can use..
Meds to use here:
linkNOW on the sulfur meds, you can try these:
Milder verson:
linkMORE potent one as it has more MG for each than the above one, but same meds:
linkYOU should be able to get these at your local fish store.. BUT USE ONLY ONE!! AND really clean this tank if you are going to have to med him in it.. TAKE OUT ALL CARBON FROM THE FILTER!!
I would however, get another tank so you dont' mess with the bio in your main tank.. It cost more to med in a big tank.. A 15-20 gal is a good size for a med tank with a JD.. All you need is a hood/ligh? filter, heater, some gravel scattered on the bottom, NOT MUCH!! And a place for them to hide in.. A NEW clay flower pot is cheap and works great.. Use the main tank water the first time..
FOLLOW THE DIRECTIONS ON THE MEDS !! Water changes with FRESH DECHLORED WATER IS A MUST!!