Do you know if there are any treatments I can do? Should I remove him from the DT to a quarantine tank? I don't know what to do in these circumstances. Thanks.
I would investigate the blindness a bit more. Try to get it to react. You can even try putting your hand in the tank and moving it slowly to the fish. If you don't create any water currents and you can get really close (or even touch the fish) it probably has diminshed sight.
There are things that mimic blindness - a fish that is severely compromised due to some issue may not react to motion and seem blind.
There isn't any treatment for blindness. I do know people have had blind fish that they are able to hand feed (but not a tang though).
Jay