I just went through this with a sailfin tang. Except I was using copper treatment. It took me two and half months to clear the ich from it. I started treating in november and just put him back in my display Saturday. Two weeks ago I completely changed the entire water. But I did not clean the filters so that way I would still have beneficial bacteria in the tank. I put the water back in exactly how it came out. With the same level of copper and same temp and Ph and salinity. The next day after that the tang was clear and is still clear
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