I'm certainly one that lacks knowledge in the fish disease department. So far, I've not had a lot of experience with my fish getting sick.
I've bought all my current fish from the same source.
I've never bought a fish sight unseen online.
The lfs I frequent caters only to the saltwater hobby.
Small family run outfit - not very large store.
He runs a product called "Sano" throughout his entire fish holding systems, and he QT's new arrivals. I've seen his back room set up several times and it's quite impressive - but again - I'm no expert.
I don't fully understand what this Sano product is, but he claims it keeps his systems free of parasites and reduces stress.
Some "natural" product he claims, and swears by it.
Could be true - could be hype. I don't know for sure.
He also has some monster UV units installed.
I've never had a problem with the fish I purchased from him, but I also consider myself lucky so far.
I did lose two small false percs that I kept in a 15 gallon high tank when I first started in this hobby. I was pretty sure it was not ick. I think it was some type of clownfish disease. I bought these two fish from another source. The 15 high was not the greatest tank I ever set up either. It lacked in some of the basic filtration and water circulation requirements.
I figure I lost these 2 percs due to either buying from an unreliable source, my own ignorance with smaller tank care, me focusing my attention on my other running tanks and neglecting some of the maintenance of this smaller perc tank - or a combination of the above.
I read here a lot that certain species of marine fish are more susceptible to disease than other's. Again I don't know if this is true or not. I just see a lot of people calling certain species ick magnets and such.
So far, I have avoided these species until I know more about them.
Someday though, when I can afford the cost and have the room, I hope to have a very large FOWLR tank, and want to keep some of these more expensive beautiful but delicate fish.
At that time, I will QT them for sure.
No doubt about it.