Regarding finding the cause of a fish illness .. .
How do you know?
Not you can't really do a blood test and cultures.
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"
Unfortunately most of the time treating sick fish is guesswork other than the simple stuff or external parasites/fungus. That's why entry quarantine is a must on an established system. Rarely do local vets even have the training. Use of a microscope and proper training is beyond most so broad spectrum treatment and guesswork for the only choice.
Entry quarantine, worming, good foods and proper tank care eliminate most disasters before they start. A few folks on the forums desire to know and have dedicated themselves through self training with trial and error to find the answers. I rely heavily on my breeder friends on TAFF II Carol and jpdevol among others. ReefkprZ here and MatsP on PlanetCatfish have also been helpful on occasion.
good luck with your fish