Ich, or more scientifically know as Amyloodinium and Cryptocaryon are the most common fish parasites in saltwater aquariums. 

The most common treatment is a copper based medication introduced directly into the aquarium and used at a strength of 1.5 to  3.0 ppm for three to four weeks. 

The reproductive stage of the parasite is referred to as the tomont. The encysted tomont undergoes repeated division, the rate of which is affected by temperature and salinity. Each tomont may release as many as 256 dinospores, which are capable of infecting susceptible fish. 

Quite often after the hobbyist has used a medication and believes that they have eradicated the parasite they seem to show up again a week or so later. This is due to the fact that the hobbyist did eliminate the adult parasites ( the ones visually on the fish ) but did not consider or allow the medication to remain in the tank long enough for it to affect the juveniles that were incubating down in the gravel and then hatched at a later time.  It is this juvenile or cyst stage that is resistant to the chemical treatment and is able to remain ineffective for at least 2 weeks in the absence of a fish host. 

Many hobbyists include various invertebrates such as Shrimps, Crabs and Seastars in their systems. Invertebrate do not tolerate copper at any level ! So what can you do ?   Many public aquariums, and Tom Frakes of Aquarium Systems, recommend dropping the salinity of your aquarium water. 

Aside from the fact that a fish can have hundreds of parasites on its exterior, there are also hundreds that make their way onto the gill membranes, where they inhibit oxygen transfer, and into the interior. A parasite tends to draw  moister from its victim which results in dehydration. 

The truth is that a fish in saltwater is normally being dehydrated even without parasites. The  simple fact that they are in a highly concentrated salt environment which has a tendency to draw moister from their bodies via osmosis. Fish are having to constantly replenish  themselves with moister. Now compound this normal situation with the addition of a parasite who is drawing more fluids from the fish. 

According to Tom, these parasites can not live in salinity's lower than 1.015 to 1.017. By dropping your salinity level down to this point you will also lessen the effects of dehydration. Additionally one can effectively treat a tank and never add any medication to it at all ! 

How fast can you drop the salinity ? I have been quite successful by replacing 5 to 10 %  of the aquarium water with fresh water on a daily basis. Once I  have reached the 1.015 to 1.017 level I maintain it there for 2 weeks. After that point I begin doing my normally scheduled  water changes with saltwater which allows the salt level to rise back up slowly. 

The overall benefit is the fish are less stressed and are able to deal  with the parasite situation more effectively. Additionally one doesn't have to remove the invertebrates from the aquarium ( Note: Not all invertebrates will tolerate low salinity's, the invertebrates I have referred to are cleaner shrimps ).