It is permanent sorry buddie. He is blind. There is no cure he is blind.
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Are you saying this happened while you were away?...the water in the tank must have taken a turn for the worst.If things are addressed soon enough this can sometimes be reversed but sadly that does not look to be the case with this one....If the other eye is fine the dat can still live a relatively normal life.
Well at least it's not completely blind..Dats are pretty hardy but their eyes can be sensitive to poor quality water(high ammonia and probably nitrates).What do you feed it?