MrTuskfish
Team RC
In line with what Bill is saying above, many (most?) aquarists rely on visual symptoms on the fish to diagnose parasites. Better technique would be to learn to read the fish's behavior. Flashing, reclusiveness, fast breathing, hanging near the surface or water returns, or sensitivity to light are all precursors to a fish displaying visual signs of a parasite. Visual signs, in the case of brook or velvet, may actually be too late. Also, many of the ich deaths reported are more than likely velvet related; ich, because of the exponential character of the life cycle usually takes longer to actually do a fish in.
Yes; a lengthy, up close observation period of new fish is vital; even if nothing bad is discovered, and it usually isn't.
I don't know how many times I've heard that a QT a stressful place----quite the opposite. A proper QT is probably the least stressful place a fish will ever experience. Nothing to bother him and not thousands of predators lurking nearby to eat him.