hmmm. one day i introduced some very small baby cories into my tank which already had adult cories and angelfish. as soon as the first baby was in, one of the angels came down to investigate, thought the baby was food, and sucked one of its eyes right out. maybe this is what happened here. you have a lot of fish in the tank and it's possible that if the angels have decided to breed, they are becoming aggressive towards the other fish.
was that cory acting strangely before his eye popped out?
any physical symptoms on him or other fish (sunken bellies, inactivity, white stringy feces, etc)?
how long ago did you get it?
any other recent additions?
any other recent illness?
aamonia/nitrite/nitrate?
water changes - how much and how often?
i am sorry for your loss, but i do not believe that putting the fish down was the right option as far as trying to protect your other fish. i can completely understand though that you were propbably very very freaked out by what you saw. i know i was very agitated when my little cory got his eye eaten.
even if it was a parasite or bacterial infection (though there would have been signs of it beforehand) it would already be in the tank well before the eye incident and in all likelyhood every fish would have already been exposed to it. next time (hopefully there won't ever be a next time) isolate affected fish in a hospital tank and then take steps to determine the problem/illness before resorting to euthanasia.
