I have used a product called ich attack. It worked pretty well for me. (in my 180 display tank). followed instructions for dosing, but dosed that amount AM and PM.
Due to the size of my tank, I had to get a gallon (or two) of the stuff. It worked well and my fish (that I would have sworn were all close to death) all pulled through. It stinks like crazy and I did not run the skimmer while I used it - but it did work for me.
I only used it the one time. (well 2x daily for a 3-4 weeks) but you know what I mean. This was about 2 years ago and tangs are all still doing well. Corals did not seem any the worse for wear....
Michael
I will say for a large tank - quarantine would have been much cheaper - but then again - most of these lessons work out that way....