my big iridescent shark is now blind and...retarded from his callous build up. i put plants against walls and now, he so dumb he can't even find his favorite pellets. im bringing him to my friend's pond cause he wants him for some reason i can't understand. he was having problems from the start with a lot of diseases and he gets spooked way easier than my other, smaller one. the other one is sad now, so he needs a new buddy...what should i get him?...it has to be under 15" at adult size.