Tangs are very prone to ick in general, and powder brown or other tangs in the genus Acanthrus are even more so...Another problem is there are two distinct similarly maked species on the market and they're both called powder brown. I know one of them is a lot more challenging to keep than the other one. Plus, 55 is not the most ideal environment for tangs, especially powder brown, which requires at least 100 or so....