Mike,
Thanks so much for the comments and all the feedback. Your tank is looking better than ever and IMHO is an example of what a cleaned and well balanced environment should look like. After I installed the phosphate reactor I noticed I had just the perfect place for another one, so I decided to add the carbon reactor.
I constantly have to frag those xenias, as they keep on moving to the overflow box.
Recently I moved the LR they are on a bit further to avoid that since they reproduce so often. It’s amazing how some corals reproduce when they like their environment.
I'm not sure and I could be wrong, but I have a strong feeling that even if you had quarantined the powder blue tang prior to introducing the fish to the main tank, there is a big chance he might have gotten traces of ich after being transferred from the quarantine to the main display.
I came across a very interesting article from FishVet, that explains this phenomenon really well.
"The Hobbyist will have a tank with several specimens all of which are free of any signs of the parasite. A new fish will be introduced & the following day, "white spots" will be observed in a great many cases, NOT on the new introduction, but on one of established inhabitants. This happened so often so years ago, that we made some experiments on apparently "disease free fish" (specifically Powder Blue Tangs and Yellow tangs).
When we took skin scrapings from these fish that had been healthy for more than a year we found evidence of trophonts under the skin. Evidently these had not found it necessary to reproduce & leave the fish, as no sign of disease had occurred over a long period of time
With this evidence we explained the phenomena just mentioned as follows:
A parasite by definition has a vested interest in co-existing with its host. As long as no unusual disturbance takes place, it will continue its idyllic existence, in harmony with its host. However when a new specimen is introduced to the Aquarium, often the established inhabitants become quite excited, feeling that the newcomer will in some way, take their "space", eat their food, or even team up with their favorite fish/companion. This causes some form of chemical message to course its way through the fishes system, in much the same way, as adrenalin causes us, to become excited if we become frightened. This chemical message, in some way alerts the parasite, which in effect says to itself, "Oh boy!, maybe I should get out from here, & look for a new host". The consequences are seen the next day, when it bores out from the host, leaving the telltale white spots."
The important thing as you've mentioned if that he's eating well, co-existing in harmony with the other tankmates, and showing no signs of stress. Hopefully the waves of the parasites will gradually get weaker and weaker until it's beyond of the point of concern.
P.S. I think I am going to order a new bottle of NO-ICH to keep it in the closet, since this product in not available in any LFS around here. It's good idea to have it available for immediate treatment if the situation gets out of control.