What's wrong with my fish?

[George]

One popping eye usually due to either 2 of these causes 1. physical trauma(eye scratching to a hard surface). 2. bacterial or viral infection. If you have a QT, it's better to put the fish in QT and treat it there. For physical trauma, you probably don't need to do anything special. Some people like to use epsom salt in QT for this treatment. Epsom salt doesn't cure anything. It just eases the pain.
For bacterial or viral infection, you will need to dose with respective medicine. Either way, antibiotic is a good idea because even with physical damage, there is a good chance of causing secondary infection.
Good luck.