If you don't have predators (like a hawkfish) in your tank, a pair of cleaner shrimp might work. Not all fish will visit but the bigger ones should. My hippo tang used to visit the cleaners a couple of times a day. Also, the shrimp are a great addition to the tank- mine climbs all over my hand looking for food
If you care to read a study where they examined the gut contents of cleaner wrasse or Labroides dimidiatus you can find it in the Marine Ecology Progress Series, volume 197: pages 241-246 that was published in May of 2000, Alexandra S. Grutter. They found that the largest portion of their diet consists of gnathiids which is a parasite, but it is not ich. Parasitic copepods, non-parasitic copepods and scales comprised the remaining portion of the diet. They did not find Cryptocaryon irritans in the stomach contents of any of the fish they examined. Perhaps cleaner wrasse will eat some ich, but certainly not enough to affect a cure. They are going to exhibit their natural cleaning behavior whether the subjects being cleaned have ich or not.
Anytime anyone purchased a tang in our store, we would always recommend at least a cleaner shrimp if the customer didn't have one. Again, providing there were no predators in the tank. Tangs are known to stress, break out with ich and well, you know the story.
As for the cleaner wrasse, some people don't like them. They are known for being a short lived fish, but at the store, we had some customers whose cleaner wrasse had no problems at all. IMO, for the cost of the cleaner wrasse verses a total ick outbreak, for my money, its worth the money for the cleaner wrasse in addition to the cleaner shrimp. If/when the cleaner wrasse dies, you either remove the body, or the clean up crew will take care of the remains in no time at all. Depending on the size of your tank, of course.
It's awesome to watch the cleaner wrasse work on the fish, and how the fish allow, in fact, invite the wrasse to clean them
Long short, at least a cleaner shrimp.
We had a customer bring in his Purple Tang, just didn't want him any more. The Purple Tang had califlower (?) on the lower 1/4 of his body. We put him in with 3 cleaner wrasses, and within 2 weeks, the califlower was completely gone.