Regarding finding the cause of a fish illness .. .
How do you know?
Not you can't really do a blood test and cultures.
I have a otto succumbing to popeye. (I am about to pull him from the tank).
I did salt, water changes (water was fine) and doxycycline. I could not get Marasyn Plus (recommended) in time.
Alot of threads says you need to know the cause of the illness.
I am guessing folks just look at the fish and look for symptoms or signs, but there are no "tests"