set up a quarantine tank large enough for all of your fish and slowly drop the salinity of the quarantine tank to 1.010. Do a 10 percent water change every day to control the ammonia and nitrates. Hold the salinity there for a 5 weeks. Take 1 week to slowly (very very very slowly) bring the salinity back up to 1.025. You should get into the habit of doing this to EVERY fish that you put into your tank.
PS: do not leave any fish in the display tank, as ich can only survive with a host (fish)
