Since we don't have a clue which disease you are dealing with. its hard to recommend anything. ( Protozoal,;, bacterial, vira)l_
Some fish diseases are easy to deal with, such as with the protzoal Ich parasite.
Add 3 lbs of salt per 100 galloms and if luck is on yout side in a week its cured.
If your fish have been exposed to a bacterial dsease such as Columnaris, it would be better to euthanize all the fish in the tank, and disinfect the entire system with bleach (including anything that touched the water) and start over.
The bacteria that cause Columnaria can exist in an inert state in a smudge of dry for years, and reinfect whenever water is again added.
Most viral diseases are incurable, such as Lymphocytes, your fishes immune system, will need to be strong enough to fight it off.
What are the symptoms in your tank?