Size of tank in gallons or litres.
How many fish and which type.
Water stats in ammonia, nitrite, nitrate, and ph.
The fish has dropsy and once scales stick out organ failure, only thing to do now is try epson salt baths out, but to be honest once scales stick out they rarely make it.
Yes best to isoolate the sick fish even if its in a container at the top of the tank, don't want the fish pecking at a dead body that's how it can pass on to other fish in the tank.,