my large angel has developed the same problem in one eye and it is as active as normal, no other problems in my tank, water great, ph-0, ammonia-0, nitrite-0, nitrate-10, im guessig mine is just old age as it is at least 6 year old.
No its a south american puffer, fresh water all the way. Had figure of 8s when i went to buy but knew they were brackish and not good for community tank.