If the fish were in your display you need to remove all fish and go Fallow. I recommend a period over 72 days since that is the longest known period Ich has survived without a host.
There are a few methods you can choose to treat the fish for Ich. Copper, Hypo, and Tank Transfer.
Copper: I am not a fan of Copper. I tried it out with my Achilles and he freaked out with a low dose of it. If you decide to go with Copper, please make sure you have a reliable test kit for it. Also do your research to make sure whatever fish you are potentially QTing with it are known to handle copper well.
HypoSalinity: I used this method when I let my 180 go Fallow for 10.5 weeks. Hypo is only effective from 1.008-1.009, so if you have a small QT it may make keeping the salinity within this range difficult. Online many say you can drop the salinity in 48 hours, I rather take a week to get there. It's easier on the fish and yourself. If you are going fallow, you have plenty of time anyway. I've used Hypo for 4-5 weeks with apparent success. When you bring up the Salinity it must be slower than going down in Salinity. You can bring up the salinity .001 per day. Monitor the fish for a week prior to placing them back, but if this is during a fallow period you really have no choice anyway. Be aware that you want to monitor the fish afterwards because there are in fact strains of Ich that are resistant to Hypo. Also, there is the possibility of error during Hypo where you slipped on the salinity and gave the parasite leg room.
Tank Transfer: I am testing this method out right now. You want to have two sets of equipment for this. People have used 5 gallon buckets for it but I am a fan of giving the fish room to swim while in QT. I am also QTing several large fish. You want two sets of tanks, heaters and powerheads. You can just move over the light. Matching the temperature and salinity is very important in this, don't overcomplicate it either.
It works as such:
Day 1: Acclimate fish
Day 4: Have a second tank with all new water, second heater/powerhead set up. In the morning before the light goes on, transfer the fish with a net with as little water as possible over into the new QT. Again, make sure Temp/Salinity match, check pH too if you are paranoid. After you transfer the fish you can drain the other QT disinfect it and equipment with white vinegar and let it dry.
Day 5 or 6: Set up the disinfected tank to match temp/salinity.
Day 7: Repeat
Day 10: Repeat
Day 13: Repeat and observe for a week while treating with Prazipro for potential internal worms.
People appear to like to overcomplicate things, My fish were half asleep when I transferred them and they were hardly stressed from it. I just netted them up and moved them over. The only issue I had was my female trigger decided to wedge herself into a piece of PVC she couldn't get herself out of. I had to physically push her out gently. Within a half hour of being moved over they were all eating. Tank transfer appears to be a great method for people who are impatient, and even better for species that won't handle Copper or Hypo well.
Thinks that WON'T work. Waiting it out, Garlic, Cleaner Shrimp and UV Sterilizer.
Waiting: I tried this with my fish, didn't work too well. They didn't die from Ich but after two months I said this is ridiculous.
Garlic: Food won't kill a parasite, I'm not sure why anyone thinks this is a viable cure. Yes, garlic is a food enticer for food. People say you should feed well with garlic.. you should be feeding well to begin with.
Cleaner Shrimp: Yes, I have seen my tangs with Ich swim up to cleaner shrimp. Did I see them eat Ich? No. They did nothing but possibly help clear away dead skin from the infected area. I can't comment on Cleaner wrasses, but I highly doubt they can eat away a parasite thats within the skin of the fish. If somebody has proof that a Cleaner wrasse will eat the parasite on the fish I would love a link, I think that would be real cool but I have only read the opposite.
UV Sterilizer: There are several stages to the lifecycle of Ich, that I am not going to get into. There is a free swimming stage in the life cycle of Ich. However, unless EVERY individual parasite passes through the UV sterilizer this method is not viable. I'm sure it can aid the fish, but to me its just a possible band-aid.
I am open for disputes on my comment. That is part of the online forum, to get information straight. I am commenting merely on my experience in exception to just the UV sterilizer. I have tried every method to treat it, its very possible for people to have different experiences. However, I think for anyone to comment on issues it should make sense that they have 120% certainty in what they are saying. I just don't think people have that assurance when they say "I fed garlic, my fish has never got Ich since than." Ich also attaches to the inside of the fishes gills which can lead to thinking a infected individual is not infected, yet they are. Higher respiration can be a good indication with an infected individual.
Sorry for the long comment, and typos. I am sure there are plenty of them.