True. Tbh I was more trying to put @James Barton off than anything else...I have to take exception to this concept. You don't destroy the immune system of a fish by doing QT and prophylactically treating them. Just like there is no such thing as a sterile QT or reef tank.
We see evidence of QT'd fish regaining immunity on a regular basis. Think of all the times people have thought they have properly QT'd their fish only to have a heater failure cause an Ich breakout. In these many cases, it was the immune system mitigating the parasites until a stress even caused the breakout.
Ideally, the only long term difference between prophylactic treatment and not is that the treated fish will no longer have an adaptive immunity built up. There is no reason its innate immune system cannot be very strong.
Agreed, the innate immune system can and should, be focussed on in any tank. This is your insurance policy should something make it through QT.
Feeding the foods recommended in this thread is generally good advice regardless of whether you QT or not and will help to keep your fish and their immune system in tip top condition.