There are certain fish diseases (viruses like Lymphosystis), that don't have a treatment. Only water changes and lack of stress so the fishes own immune system can work it out
There are certain diseases (bacterial like columnaris) that are difficult to treat unless it has been diagnosed and a proper antibiotic is used, (columnaris needs a gram neg antibiotic)
Diseases like ick, are easy, a little salt, a little heat.
The problem with fish disease is many have such similar symptoms , it hard to discern.
I have never been successful at treating columnaris, so if it ends up in one of my tanks, I expect all fish in the tank to die, I then break down the tank, completly bleach it and any equipment used in it,before adding any new fish.
This is why a quarantine tank is so important to use before adding new fish.
I usually quarantine any new arrivals at least 4 months (a friend of mine quarantines 6 months), and never use aquatic live foods, unless I raise them myself.
The cost of meds for one disease, usually equals the cost of a Q tank