The following is from an eatlier post of mine on ich. It may be of some help.
A similar situation occured in one of my tanks several years ago. Both the Paracanthurus hepatus(Hippo Tang) and (Zebrasomma xanthurum) purple tang survived after several infestations but spotted up a little every week or two . At this point I have had them both for over 5years and they are doing well. Other fish were lost though.
Apparently, you have ich in your tank. It may expire on it's own after 34 life cycles. It will attack new fish,however. Particularly those prone to it.
The following is from another post of mine and may help you decide a course to follow.
Uh Oh another ich thread.
I think treatment of the fish and the tank is the best option for an ich (cryptocaryon irritans) infestation. The treatment protocols are difficult and many do not choose to do them or are in a circumstance where they can't do them(such as catching a fish in a fully developed reef tank). You have to make a choice wether to treat or not treat. The following should give you some information to help with this decision.
Treatment:
There are only 3 proven treatments for ich. Copper,hyposalinity or the tank transfer method. All of them require removing the fish from the display tank and treating in (a) separate tank(s).
I personaly prefer copper treatment or tank transfer since Hyposalinity for a prolonged period is ,in my opinion, quite stressfull and has been ,in my experience, ineffective.
When ich is in the tank it is very likely all of the fish are affected wether or not they show obvious symptoms since many parasites host in the nostrils, mouth and gills. Fish with lesser infestations that are not visible probably have a degree of immunity perhaps from prior exposure(82% of surviving fish that have had significant exposure develop some immunity). So it is necessary to remove them all and treat them.
It is also necessary to leave the tank fishless for a minimum of 6 weeks( some cysts have survived for 72 days) to be pretty sure that all of the parasites have hatched and perished for lack of a host fish. Otherwise, new fish including th one you may treat will likely be attacked.
There is a lot of information available on this protozoan parasite and a google search or a search of the fish and disease forum on RC will help you understand a lot about cryptocaryon irritans and it's treatment.
No Treatment:
Hope and cope strategies are often employed. Some fish can and do survive an infestation, Some don't. In either case the ich will be in the tank for about 11 months unless it is particularly virulent or does not hold true to form with studies on its life cycle. It is believed that a single strain of ich can reproduce a maximum of 34 times (over a period of about 11 months). After which it expires even when a host is available. This is attributed to cell aging in the strain.
During this period new fish are likely to be attacked and the fish in the tank already will have some level of infestation seen or unseen usually every week or two.
Manging water quality with an emphasis on steady temperature and salinity and good nutrition are important to keep the fish healthy and to give them the strength to cope.
There are a number of "reef safe treatments" on the market. In my experience none are effective teatments and some are not effective or reef safe.
Other things aquarists do in an effort to help the situation when treatment is not chosen include: uv sterilization, cleaner shrimp, neon gobies, garlic soaked food,diatom filtration. None of these will cure ich in my opinion but they won't do any harm either and may have some benefit when a decison is made not to treat.
Good Luck
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I cannot for the life of me figure out what is stressing my fish.
