Asylumdown give you great advice, I agree. Keep in mind that Ich (note proper spelling lol) is kinda like a fish version of human influenza - most people/fish get it occasionally, and it's usually overcome by healthy individuals. I would take Asylumdown's advice and sit back to watch the tank for awhile.
Imo, attempting to make a tank completely Ich free is fairly easily accomplished by following a 12 week quarantine with hyposalinity and keeping the tank fallow for that time, plus same quarantine for incoming fish. I don't think it is something that is that imperative, and 12 weeks is a long time. Do I think it is worth it for a Biocube? No. Do I think it is worth it if you're setting up a big tank and planning a bunch of very expensive (and usually less common) fish? Yeah, that might be a good idea. Do many people do it? I can only think of a couple people I know that follow this VERY strictly.
Imo, there is no Angelfish nor any Tang that would be suitable for a Biocube - they all get too big and too aggressive, so it's tough luck that the Angelfish was returned. If the tank hums along without a hitch for 3-4 weeks, consider adding a small fish like a Shrimp Goby (with a cool pistol shrimp) or one of the small species of Blenny like a Tailspot, Pictus, or Tiger. Not one of the big Blennies like the Lawnmower or Bicolor.
I like hyposalinity in quarantine for new fish. I do not recommend it as a means of treatment since hypo doesn't work fast enough for a serious Ich infection, and I don't recommend treating mild Ich infections, so that leaves a moot point there.
I also don't like copper treatment - for anything. Chloroquine phosphate is a much easier and safer method of treatment in place of copper, although more difficult to obtain (purchase ahead of time!).