most, if not all fish, can carry ich to some degree. Only when under stress, does their imune system break down a bit and the ich just "explodes" on you. This can be fatal to each and every fish in your tank, so be careful.
I would consider the purchase of a UV
sterilzer, which will kill the parasite when it is in the free floating stage.
You can do freshwater dips and other chemical dips to kill the ich when it is on the fish
You can do nothing about the parasite when it is in the cyst stage in your substrate. I hear of people using copper to control, but I do believe that the cyst in the substrate is nearly indestructable.
If you add copper to your tank, be advised you will never be able to add any inverts. The copper will leach into the LR
and sand, and if the tank is acyrlic it will even leach into that as well. The copper will be released back into the tank for years to come.
I know someone on here is going to beat me up for this, but this is what I would do, and have done myself for my blue tang that I have had for over 5 years now (so I must have done something right)
If you want tangs, invest in a UV
sterilzer. Tangs are just bad for getting this disease. Do your research on these things, some they sell at LFS
are not for salt tanks, only for ponds. Google UV
sterilizer and read, read, read before you buy. I would suggest a model that is easy to clean as well, some are impossible to work on.
Get a cleaner shrimp or two - they will pick the parasites off the fish
If you have nothing else that will eat them, get (cleaner) neon gobies, they do the same job and are cool fish to have as well. If you get enough of them, they will breed in the tank.
Avoid cleaner wrasse, they don't do well in home aquariums.
Do freshwater dips and/or other medication dips as directed on the bottle. If you go with freshwater dips, get some RO
water the same temp as your tank, and dip the fish for a few minutes. You must watch the fish, as sometimes in a weakened condition from the disease they can't handle the dip as long as on a previous dip. Just watch him closely. If he starts 'breathing' super heavy or lies down on his side, remove him, he is done.
In the future, set up a plastic 10 gallon from walmart and use it as a quarantine tank. You can get a complete system for $40 or so. Leave the fish in there for at least a couple of weeks and you could avoid things like this, and you could have treated the fish by himself in the quarantine tank.
Good Luck - my blue got ich bad when I moved my tank from one part of NC to where I am now, and I had him for 2 years at that point. Stress is what brings it out, but with tangs it seems it is really always there.