Is My Rainbow Shark Blind Or Sick?

[melindakm]

For the past few days my rainbow shark has been acting odd. He's normally chasing other fish and swimming freely. On closer inspection he seems like he might be blind. He's staying close to the side of the tank and when he touches anything like the thermometer or tubes or even bottom of the tank he jumps as if didn't see it. He swims but doesn't leave the side. Aslo if I wave my had close to his he doesn't acknowledge me. Can a fish go blind or is there something wrong in the tank. I tested the amonia and there is none and the heat is about 80 degrees F.

 

[WILDER]

How many gallons or litres is the tank.
How many fish and which type.
Water stats in ammonia, nitrite, nitrate, and ph.

Does the fish look pale or darker in colour, being lethagic and listless, is the fish eating.