what do you feed them, do they all eat the same food, i know you said that you have been feeding them the same food for the past few weeks, i know a guy who bought some blood worms, fed them to his fish and the next day they were all dead, he reackons that the blood worms had been thawed out then re-frozen and sold, food poisoning, cause his water was fine as well.
I dont know, just an idea?