Used to be most of the time I QT my newcomers. The few times I've didn't do it, about half of those times, I've regretted not doing it. Sucks losing members of an established tank to newly introduced pathogens (of one type or another). Last time was final straw when I dumped 2 new goldfishes into my goldfish tank, turns out one was infected with ick, fungus and some kind of bacterial infection that causes severe bloating. Lost half of my prized goldfishes (some of which I've picked up years ago), many are very hard to find (mostly extremely longfinned versions of the various types - ranchus, lionheads, ryukins and one celestial with veiltail caudal and lower fins). Luckily my pure black lionhead, pure black superlongfin moor and my pure black veiltail ryukin was out of the tank (club show) at the time.
Now I try to QT ALL newcomers. I normally have 2 QT tanks, a 5g and a 10g, depending on the size of the new fish(s). I also have available a 30L on standby for those monster newcomers (I just picked up 2 11" arowanas that just went into this tank). All three are bare tanks with power filters and airstones. The 2 smaller tanks usually have water in them at all time, so that the water's fully aged and cycled when a new fish comes in.
I do minimum 2 weeks QT, and I only medicate with pimafix and melafix automatically. I treat for other specific pathogens ONLY if they are observed or suspected, in which case I'd then increase the QT period as appropriate. It's better safe than sorry to lose your prized established tank. It also helps if you observe your potential aquisition and pass if it shows signs of ANY problems. A problem avoided is a problem you dont have to deal with.
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