Most likely some fighting going on when you can't see. Maybe during the feeding, or the sharks picked out the eyes. Goldies don't usually attack eachothers' eyes like that, not to mention that they have rather round noses so it'd be tricky to pick out an eye in the first place!
Maybe you should try and get those sharks out of the tank? Since you know they're not supposed to be there. You know, just as a safety measure!
My baby pearl scale lost an eye a couple of months ago. I separated him and added salt to the water, he healed up without problems, but seemed to be very sad being alone in the tank, and he slowly lost control over his swimming :/ One evening I saw him floating on the side so I did a large water change and added some Epsom salts but he died the next morning, sadly
I'm not sure how he lost the eye either, considering that he was SO small!