They are essential, take it from experience. When I first started I read and knew how important they were but ignored the entire process. This went well for the first 3-4 months, then got a batch of clownfish that developed an infection that killed them within 24 hours. I thought Ok, so they died now I will be OK, then my tang started getting a slime over its face and started thing oh crap, it must of been velvet and now the rest are going to die too. I literally spent over 7 hours, tearing down the entire reef to catch all of the fish and put them into a QT. At that point I realized my mistakes and have tried real hard to not duplicate them again. There are certain fish that do not do well with QT and those you may have to take a chance with, such as Leopard Wrasse's, Regal Angels, mandarin fish etc. these fish are notoriously difficult to get eating and the best thing if you get a healthy looking one is to get them into a reef environment ASAP, but you are taking the better of two risks (guaranteed dead fish from starvation or a calculated chance of getting a infection of some sort in your display). Since these fish are best left to people who have had their tanks up for a while, the risk is something we all think about before buying a species like that. QT is a must if you don't want the hassle of trying to catch fish out of your display, and believe me tangs and blennies are not the same as guppy's and mollies
. These saltwater fish are much better adapted at avoiding getting caught. I could not believe how difficult it was to remove just 6-7 fish from a reef tank.
WreckDiver
aka: CyBrSuFr