Ok. Now that we know so far that water quality is not a factor in the case, we can look at other things. It's always good to test the water whenever there is something wrong to either confirm or deny problems in that area. Keep feeds down...keep to your normal maintenance routine, but do service the filter. Sometimes bacteria and fungus can get a foot hold in the gunk that get trapped in tubes and compartments without really making a noticeable change in nitrate levels. Especially with a good filter.
Still could be caused by disease causing bacteria and fungus. You still would rather isolate the fish from the general population...just in case. It is contagious. Once you are positive on the cause of this popped eye, then appropriate medication can be applied.
Observe the fish closely. When lights are off, you can use a red light to observe the fish without disturbing their night time cycle. It is also possible that the fish could have banged it's eye. Other than the popped eye, are there any scrapes, scratches, tears or faded areas around the eye? Perhaps indicating obvious injury?
Regardless of what the popped eye originates from, you can help the fish by adding stress coat. Best kept in a hospital tank fitted only with a sponge filter, heater, thermometer and a plastic hideout. Bare bottomed.
You could treat the fish with Methylene blue...a disease prevention, if the problem is due to injury. Use Malachite Green...a disease treatment if the problem is disease, but treatment with any chemically based medication should only be done in a hospital tank to avoid damaging plants and other fish. MelaFix (preventative) and PimaFix (treatment) are natural alternatives for situations where the animal cannot be isolated and will not harm bio, live plants or uninfected fish.
Let me know how it goes
We, as a people, know so much more about outer space than we do about our own oceans. This lack of knowledge can very well spell the dangers that lay in wait for us.
The oceans surely would swallow us before a rock comes down to smite the planet of it's life.