Size of tank in gallons or litres.
How many fish and which type.
Water stats in ammonia,nitrite,nitrate,and ph.
Sounds like swim bladder does the fish look bloated.
Sure it's litres and not gallons, the tank is severely overstocked, convicts need 30gal and no less.
Are you sure them stats are that.
If the fish is upside down he's not going to make it sorry.
I would check your tank and stock out by a thread in tropical chit chat, good luck.
It cant be ammonia posining if his ammonia is 0.