I was battling ick for a very long time. Then I lost my powder brown tang and shortly after the powder blue tang. The all had Ick. What I did was start to make my own fish food. I literally dropped a drop 10 to 15 drops of the Kent garlic. Over my home made fish food. The fish go crazy over it. And voilà I haven't had Ick in the tank ever again. I will be adding more fish soon now that I have that under control.
FEED, FEED, FEED. fat fish are healthy fish, that's been my experience. Big temp swings need to be stabilized, and keep good water parameters . Then hope for the best. Recently brought in a blue hippo and yellow tang from a tank breakdown. Both fish had ich, tossed them in and proceeded to feed them and fatten them up. No more signs of ich and other fish a still healthy. Have done this in the past with other unhealthy fish that had ick/ HLLE from hobbyists that let there tanks go.
This did not work for Uronema or Brooklynella, the infected fish ended up dying, however the existing inhabitants did not get wiped out as well. So, the lesson learned there was even if a LFS says all the fish have been quarantined and are healthy doesn't mean anything.
I personally tried the "managed & feed heavy/garlic/UV" approach in the past and failed horribly. It was so sad to see the fish die one by one over a short period of time and that was on a lightly stocked 180g. They would just stop eating and die.
now everything that goes into my tank goes through a full qt. it is more work but at the end of the day its so much more enjoyable to know that your fish are healthy and you don't have to worry about an outbreak.
I would suggest you take the fish out and do tank transfer method on them to rid of ich while keeping your main tank fishless for 8-10 weeks. Also make sure you are not adding anything wet to the tank that can harbor ich or any other pests to your dt (rocks, corals, inverts, etc).
good luck.