I've got 8 black neons, 7 rummy noses, 4 cories, and a betta in a 29 gallon tank. I noticed one day that one eye of two black neons was missing. I didn't see any signs of disease or anything. What is a valid explanation for this? I thought maybe it was the betta or something but the betta gets PLENTY to eat(darn thing looks constipated).