If your tang is infected all your fish need to be treated in a separate quarantine tank and your tank needs to be kept fish less for around 10 weeks to kill the parasite in the tank off. First I'd make sure it is ich. Pics would help.
Ich is treated with either copper or hyposalintiy.
If you opt for copper make sure you have a compatible test for copper. Follow the directions on the bottle. There are chelated copper preps like copper safe or there is cupramine. When using copper to treat fish do not use any type of water detoxifier like Prime in the tank.
I prefer hyposalinity. With this you need to be able to measure the salinity very accurately with a calibrated refractometer. Over the course of several days slowly lower the salinity to 1.009. Too fast will hurt the fish. It has to be kept there for around 4 weeks. You will need to test the salinity daily and make sure it doesn't go above 1.009. This method is less stressful for the fish and if necessary you can use things like prime to help with ammonia. after the treatment period you very very slowly raise the salinity until it is back at 1.025. You will still need to keep the fish out of the main tank for several more weeks or there was no point in treating them at all.
Once both your fish and tank are ich free, you need to quarantine all new fish for at least 4 weeks prior to adding them to your tank.