I'm not super genius, but I do pay attention to a fish before I buy it, or if I buy it from an online retailer, I'm really picky about who I buy from. I pretty much only buy online from LiveAquaria/Divers Den and PhishyBusiness, but I have purchased from Vivid Aquariums without issue also. If the fish looks healthy(locally), I wil put it in QT and begin watching. My main concerns are ich, velvet, isopods, flukes, and intestinal worms. Keeping the fish in QT for a month ensures I get to see how it does before possibly exposing my other fish anything it might have. Flukes, brooklynella, and isopods can be treated with formalin baths. The link I gave you earlier will have info on formalin baths. I've done them in the past with no issues. Ich is treated with hyposalinity as its the easiest way for the fish. I use a ph monitor and drip sodium bicarb to the tank to keep ph and salinity stable. Hyposaline water is not stable ph wise.
There are various meds which can be used to treat for worms/intestinal parasites...jungle actally makes a food with the meds in them.
Its not hard to QT, and the benefits far out weigh the hassles. I had some problems with AEFW's and lost all my acros....shortly after that, I got a pair of white bonnet clownfish and the promptly came down with Brooklynella. After that, I decided to QT everything.
BTW, that wasnt police brutality, and I'm not the tang police either...
Nick