I dont want to start another war about how large your tank should be in order to house a tang, but just a thought. A buddy of mine had a yellow eye kole in his 60 gallon hex, same thing was happening, water parameters were great and everything, and yet he still looked stressed, thrashing along the side of the glass, had awful scratches and discoloration under his eye around his cheek. We transfered him out to another guys 90 gallon, and within a week, everything cleared up. Although koles dont require the amount of roaming room other tangs do, you never know. Don't know if stress was the culprit for thrashing or not, but just throwin in my 2 cents