.. He looked fine, so I replenished the stock in my dt, ...
It could be ick, you should get a UV sterilizer and run it in your tank. I recently had a ick outbreak and it killed most of my fish. Only a few fish was able to survive; Yellow Tang and Flame Hawk. I added a Alantic Tang and he caught ick the next day, I added the UV sterilizer and fed them brine shrimp soaked in garlic with seaweed strips (soaked in garlic). They pretty much healed and there was no sign of ick after 1-2 weeks.