Like Paul B I have never QTd a fish in my life and or the last 25 years or so I have not had any disease in any of my tanks. Lucky you may think..... I think luck plays little part in it. I have most probably introduced the likes of white spot on more than one occasion and yes although for a few days after introducing a new fish I have seen flicking I have never seen any spots ever. I don't treat my tank unless you call the fact I use Oxydators as treatment or you consider the foods I feed and the fish oil I add to my foods some of which are home made. IME healthy fish either don't catch many of the diseases we hear about or they fend them off well before they become a major issue. Controversial, well yes but my fish live long lives and many spawn regular for me. The health of my animals speaks for itself however but I must be doing something wrong for my fish not to catch the likes of white spot.
What kind of food are you feeding them and how do u give them fish oil? In the food?