What is the temp currently? eSHa Oodinex is a wide range treatment that should help you in the short term, despite what some will say it is completely safe for invertebrates which you may have on the live rock. Turn your temp up by 2 degrees celcius but no higher than 27- this will speed the process up and the parasite causing the ich are more vulnerable to the treatment. Also - remove filter carbon and turn your skimmer of for four to six hours. IF you have a uv
steriliser, firstly turn it off and then after there treatment get a new bulb as the one in there may not be working. Aside from the advice already given (which is purely your choice to follow but it has worked for me in the past) I would advise getting rid of the tang and forgetting about any tang in that size tank. Sorry if it's not what you wanted to hear. Good luck!