I recently battled ich with my Kole Tang, and got a lot of helpful advice from people here. This is what has worked for me:
Immediate water change. The sooner the better. Make sure the fish are still swimming and eating at a healthy rate. If not, you may consider getting them into a quarantine tank. I've been taking a full cube of frozen food and sticking it in a shot glass. Then I put just enough garlic extract in the shot glass to cover the food and then put in a quick splash of vitamin c. I swirl it around a little bit, then stick it in the fridge for 6 hours or so. Come feeding time, my tang turns into a Saltwater Swine, gobbling up as much as he can. I believe the vitamin c in the food has helped his immune system, and it doesn't hurt my clowns either.
I still notice little bits of ich on him here and there, but its far better than what it was initially. I have a fire shrimp that picks off the dead tissue on the fish anytime they're near each other, so that has helped too. My tang is swimming and eating normally, so I haven't been too concerned with him. But I do still keep a watchful eye on him. As long as your fish are behaving like they should, just keep up with you water changes and supplement their diet, and hopefully they'll be fine.
This is what worked for me anyways. Just trying to pay it forward.