If this is already an established reef tank (with corals, inverts, etc), you'll need to remove ALL fish from it...have fun with that...lol, and have a QT tank large enough to keep them in for at least 8 weeks, as mentioned above. The QT tank will slowly be brought to an SG of 1.009. It needs to stay at that level for 8 weeks and then slowly brought back up to 1.025. In the meantime, in the fishless display tank, without any fish hosts, the Ich will die. In the QT tank, the hyposalinity will kill the Ich. Do regular water changes in both tanks during this time.
Sometimes, it's easier to remove corals and inverts and do the hyposalinity treatment in the display tank. Keep in mind that this will have a damaging effect on your live rock as well, but it will recover. When this happened to me, I had a second reef tank and was able to remove all corals and inverts from the infected tank. I then did a 10 week hypo treatment in the infected tank. What was funny is that in the 3rd week of treatment, my featherdusters went NUTS and grew all over the tank. Unfortunately, they ALL died off in the 4th week...lol. The live rock did recover though and the feather dusters even grew back.
Keep in mind that Tangs are notorious for getting Ich and IMO, they need specialized QT treatment when first purchased. I only have a Yellow Tang at present but it went through it's own 8 week hypo treatment as soon as it got home from the LFS.