I agree with Baji ^^^^^^ my roomates and I had a silver and a jardini in college.... they both developed it in one of their eyes around the 12 inch mark, when they both decided that cruising around the top of the tank was better (we introduced a couple of bottom feeders). I have no idea what causes it, but we speculated that they both got it from continuously watching the fish swim underneath them...... We didn't mind it too much, as we viewed it as a natural adaption to their environment(unfortunately caused by us). the tank was an 8ft long by 2 ft wide by something or other tall.