RBC,

Could be a condition in fish known as "exophthalmos"(bulging eyes).It's sometimes accompanied by an enlarged abdomen and raised scales. It's a symptom that can be caused by various bacteria and viruses. However, like Eastland says just because one fish had problems does not mean the rest of them do.

There are always some fish in a population that are more prone to disease or become prone when they get old.

Rapidly rising or flucuating water temps can cause fish to be more vunerable to pathogens always persent in the water.

Only other thing that I know of that would cause bulging eyes is "Gas Bubble trama" caused by supersaturation of mainly nitrogen, but that is unlikely in your case unless you are pumping in large quantities of well water. In rare cases it can happen just after ice out if a pond freezes back up and thaws repeatidly.

If pigs could fly bacon would be harder to come by and there would be a lot of damaged trees.