Use a quarantine tank! It's as simple as a 10 gal tank with a heater, some type of filter, and some plastic plants for cover. Put you damsels in there now and treat with a saltwater ich medicine. Leave your main tank empty for at least a month before adding your fish back in it. But, don't treat your main tank with medicine. There's no guarantee the ich won't come back though. Once it's in your main tank it's hard to get rid of. I've been using a qt tank for years and have never had a problem with any parasite in my main tank, although a few fish have been sick in the qt tank. As I said before, a qt tank is one of the most valuable pieces of equipment you can have.