Just a thought about the dark fish-- the description of swimming lazily while other BGs are feeding, aggressively attacking the pellets. I've seen similar behavior of a few fish that were stressed and died within a few days in my pond this spring. I'm not sure why the LMBs don't make quick work of stressed fish, unless they can sense something undesirable about a fish that is sick or stressed?? Just another possibility other than blindness. The fish that stressed in my pond seemed oblivious to their surroundings. By that I mean they swam erratically and at times swam right into the edge of the bank- literally bumping into it. Almost like they were blind. In several cases turned a darkened coloration too. In my experience this spring, the fish that stressed don't recover. If this is a blind or stressed fish, hopefully it's the only one you find.