
That is indeed what we call Pop-Eye. It's a bacterial infection in the
eye. You can remove the fish to a Hospital Tank where it's treated with
chemicals but it's a lot of trouble and sometimes just leads to fish death because of the stress of moving an already weak fish to a new home. Also, a Hospital Tank is not
something you just set up like a goldfish bowl. It takes months to set
up an effective HT.
The other option is easier and the one to try first. It's Garlic Oil. This is in no way
a cure, but it is the best natural remedy I have found to help prevent
various illnesses from killing the fish. We typically use it to prevent the Ich parasite, but Garlic is often effective against bacterial infections. Look below in the WMAS Reefkeeping Tips for the Tip called "Secrets of Adding New Fish, Fish Training and the Miracle of Garlic Oil".
Another thing to check is the health of the aquarium. If you would like to post a pic and tell us a little about the tank we can give you a free health evaluation.

P.S. Sometimes Pop-Eye goes away on it's own and I've seen fish live for a long time with it.
Edited by Mark Peterson - June 10 2012 at 8:55pm