There are ich medications that are reef safe, but you have to use so much of it and I don't think its effective.
Since you're going though what I did way back when (I had ich twice in my reef), and assuming you believe me in regards to fish immunity, I do have a solution for you. Its called herd immunity.
When ich wiped out my tank, I lost everything except for 1 species of fish. Within 2 months of adding a single ich infected fish, ich population bloomed exponentially with each generation and every single fish was covered, at which point, my tangs, clownfish, even my damsels died. However, my 6 blue-green chromis survived and became immune. I then added 4 more chromis to get a school of 10 and they also developed immunity very quickly. Once the chromis were asymptomatic, I was able to add any one single fish, no matter what. So when ich is randomly swimming around, it may hit a chromis or the new fish. The ones that hit the chromis are pretty effectively cleared by its immune system and that breaks the ich's cycle. The ich that hits the new fish will all reproduce, but the new ich will be soaked up by the chromis, preventing the run away bloom.
Using herd immunity, I was able to introduce zebrasoma tangs, more clownfish and damsels. Evenually I also added my hippo tang (regal to you) which had the weakest immunity. He was a ich factory for the rest of the year, till one day, ich died out.
NOTE: I also added a powder brown tang to my sump. I believe this fish's immunity is weaker than the hippo tang. I kept him in the sump, so I could expose him to the ich, and catch him and treat him with cupramine in a separate tank in case he couldn't do it himself. (I just needed him to develop what immunity and live though each exposure) He was doing fine until one day I added a bunch of damsels to the sump also, which the ich then blooomed off of and the tang died suddenly on the first bloom, a day before I was going to catch him. My mistake, since the damsels weren't immune, and the ich bloom happened in the sump, whereas my immune fish were in the main. I then rememebered ich can only swim a short distance, like a foot.