Do you have a quarantine tank? If not, can you get one? It doesn't have to be anything fancy...just a tank with a filter of some sort and maybe a powerhead or two. Put in a few pieces of PVC
pipe for the fish to hide in. Anyway, if you can set that up, you can move the fish in there and treat them with copper. The ich will die out in the main tank in a few weeks without a host animal. You could up the temp a little and drop the salinity to speed things up a little. While the malachite green is often used in FW
tanks for ich, I don't think it's an option for SW
fish. Copper is your best bet...use it very carefully and don't ever let it get into your main tank as it will kill all your inverts.