Tangs can't kick the Ick


Reefing newb
I have a 180g reef saltwater tank that has been running for years. About 2 months ago I added a sailfin tang and a juvenile mimic tang. My lfs had both of them in the store for over a month eating and healthy which is why I took them hoping they would be more resilient. They were good for the first month in my tank but now they have had ick spots on their fins for the past few weeks and they are scratching on and off. I have been adding Herbtana for almost a month , the tangs are acting totally normal except for the scratching and they eat like pigs. I just cant seem to get rid of the ick. My lfs suggested to stop the Herbtana, add a cleaner shrimp and just feed them more times per day and basically said tangs always have ick. Water parameters were tested and they are all perfect. Shrimp has not helped, tangs wont let it clean them. My tank temp is around 75 degrees. I have read that raising the temp can help kill ick. They are acting normal except for scratching and thrashing at times but I am concerned it will progress and kill the whole tank. Any suggestions?