Here is a little explanation for you:
Hyposalinity
Please know: You cannot do hyposalinity in a tank with live rock, snails, hermits, stars, shrimp, live sand, corals, etc. Hypo will kill these organisms, and thus must be done in a quarantine tank. If you have none of these things in your tank, you may perform hyposalinity in your main tank.
When treating your fish for ich, all fish must be treated with hyposalinity, including the ones that do not currently have ich. As long as there is a fish in the tank, ich will be alive because it has a host. Even if you do not see it, it is going to be there. You will only rid your tank of ich when you remove the fish, put them in hyposalinity, and leave the display tank fishless for 6 weeks.
When you do hypo, you will need a refractometer so that you can ensure specific gravity accuracy. You will also need a pH and alkalinity test kit, and a buffer in case either of these levels begin to decline. I prefer Kent's Super dkH buffer.
Hyposalinity (hypo meaning lower) is when you lower the specific gravity of your tank to 1.009 over the course of 48 hours. I would always recommend that you do it slowly, as to not shock your fish. One way of bringing the salt level down is to do a series of small water changes, replacing the water in the tank with RO/DI water (without salt). When you begin to lower your specific gravity, you need to pay very close attention to pH and kH. These things may begin to drop, and you need to be ready to buffer them if they do indeed fall. Keep the fish in hyposalinity for 3 weeks. Begin counting the 3 weeks once you see no ich on the fish. While the fish are in QT, pay close attention to nitrates as well. If this level begins to rise, a water change will definitely be needed. When doing a water change, always be sure that the pH, kH, salinity, and temp is the EXACT same in your water change water as in your hyposalinity QT. After the fish have been in QT for 3 weeks, SLOWLY begin to bring the specific gravity back up. Do this over the course of a week. Make sure to do it very slowly, only bringing it up a few points each day. If you do it too fast, your fish will begin to stress out, as they do not tolerate fast increases of salinity.
After the salinity is brought upto the salinity in your display tank, leave the fish in the QT for 2 more weeks to ensure all the ich in the display tank is dead. This will total 6 weeks that the display tank has been fishless. Without fish in the display tank, ich will slowly begin to die because they have nothing to keep them alive. Ich needs fish to keep their life cycle going. Inverts cannot be a host to ich.
After 6 weeks, all ich should be gone in your display tank and your tank will be safe to add the fish back in.