Q. Last year I set up a fish tank for the first time. I was given some African cichlids and Corydoras catfish fry by a friend of mine who breeds them. All went well for about six months. Then I noticed the eyes on the corys were gone. They're still alive and seem fine except, of course, that they're blind. How long will they live this way and what went wrong?
A. It sounds as if you've had your first lesson on fish compatibility, something most new aquarists learn the hard way (and some aquarists never learn at all). It might be considered an addition to Murphy's Law — in the aquarium all fish are compatible, although sometimes only briefly.
What I'm guessing (you didn't say what size your tank is) is that your African cichlids became sexually mature. As a novice fishkeeper, some people may tell you that cichlids are "mean," and with your recent experience I wouldn't be surprised if you believed that. Actually, cichlids are territorial. In the confines of an aquarium, particularly a small one, they become more aggressive because they're trying to establish and protect their territory.
I think a little cichlid background might help you to understand what happened to your Corydoras. They were the interlopers. They didn't respect — because they didn't understand — the threat displays of the African cichlids. The armor plating of your Corydoras (they don't have scales) was probably some form of protection from the cichlids. However, their eyes were vulnerable.
On a happier note, as you've already discovered, your corys are not hampered by their blindness. They can live and even spawn without eyes. My suggestion, however, is to return them to the person who gave them to you for use as future breeding stock.