I gotten Popeye on a Koran Angel which I think was a secondary infection to Fin Rot. Or vice versa but I think it can be multiple factors, Stress, scratched their eye on LR, etc. Most likely stress. If you can, put the fish in a QT and use Maracyn 2 for marine tanks. Works great and cured my fish in 1 treatment (5 days) also cured the fin rot.