I have three goldfish in a twenty-gallon tank. I know it's not the best set up and I would like to move them to a bigger tank when I get the opportunity, but they seem happy and healthy enough. I have had them for about a year now, and recently I have begun to notice that two of them started developing pop-eye. Whereas the third fish in the tank had also developed pop-eye, his was confined to one eye and appeared as a white sac behind the eye. In his tankmates' case, their eyes are slowly protruding out of their sockets.
I change their water regularly about every four days and use API Stress Coat with Stress Zyme to ensure their water stays clean for longer. I also test the water for Ammonia every other day and find no problems with it.
In this article, Meredith Clawson mentions that:
Stunted goldfish may sport permanent bulging eyes as a result of disproportionate growth.
In another article, she talks about stunting in goldfish, and the implications on their health, and she points out:
Some slightly different proportions such as larger eyes seem to be often seen with stunts, but that doesn’t seem to be harmful to the fish.
However, many article on goldfish popeye mention that when popeye is caused by bacterial or parasitic infections, it usually does not show up with other symptoms until much later, which appears to be the case with my fish.
If the popeye is due to an infection, I would like to know and treat them as soon as possible. How can I tell if my fish are developing popeye due to stunting or due to some sort of infection?