Well there's a lot of debate regarding Tangs, spots, and Ich. As littleme says, true Ich infections are terrible and can do a number on a tank quickly, littleme and I alas speak from experience
. For years Tangs have been known as Ich magnets and people seem to notice that they "catch" ich when hungry or stressed. This has led to the theory that Ich is present in almost all marine systems and as such shows itself when a fish's immune system is comprimised via stress.
Often times Tangs when they show Ich are actually able to beat it if it's just a few spots. Heavy feeding usually helps the fish beat the parasite itself. However sometimes when the Tang can't beat the infection itself, or it's already moderate/severe you can have a full-fleged outbreak. Alas as with anything in this side of the hobby there is no black or white, just a whole bunch of gray area
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If you do decide to treat, there are really only two methods that work. First, QT ALL fish into a tank with a small amount of live rock, a sponge filter, a small powerhead, and smoe plastic pipe/decorations for cover. Do not use sand. Then slowly over the course of say 24 hours drop the specific gravity to around 1.009. Once the fish stop showing signs of Ich, leave the QT tank at 1.009 for a week. After which, slowly start raising the sg over the course of a few weeks. Just allow evaporation to take place and topoff with 1.026 water until sg gets back up to 1.026. At this point it has probably been a month with the display tank without fish. IMO, resist the temptation to add fish back until the display has been without fish for 2 months as the parasite can lay dormant for that long.
The other method is to QT the fish in the same manner as before but leave the sg at 1.026. Then you treat with copper, dosing as-needed to keep a level of 0.2 to 0.3ppm of copper. Repeated dosing will be necessary in the beginning as the piece of LR will absorb copper at first. Keep the fish at this level of copper for 1 week straight to kill the parasites. Under-dosing may not kill all the parasites, and overdosing can kill the fish so be careful with this method. Copper is however exceptionally effective at killing Ich. Once the week has passed you must again leave the display tank fish-less for 2 months to ensure all parasites die there without a host. Obviously stop dosing the QT tank and do a series of waterchanges to bring the copper down to <0.1ppm.
Ich is a PITA to try and eradicate and I realize what I'm proposing is less than ideal, but if the Tang does start showing spots beyond a handful or so, it might be a good idea to go through the pain of doing it. If another fish shows ich, go through one of the treatment methods immmediately, do not pass go
HTH
the tang is a original member ov my 1year old tank, there is corals and critters in the tank, i'd be gutted if i lost anything not just for money value but for all the hard work i've put into it 
ta for your advice littleme
wot do you think ski? yor comments and advice are always top draw 
are colourful and lively. 
