I recently went through this. There can be many causes to pop eye apparently. It could be a physical wound (scratch, etc..), bacterial infection or a fungus. I did a big water change to make the water quality was good and then I treated the tank with Maracyn 2. I believe it is a 5 day treatment and afterwards my gourami was fine with no reoccurence. I also kept the lights off for the treatment duration and did no water changes (per the instructions with the meds). This did not affect the water quality to any large degree (ie no ammonia present).
