I am not sure. During certain times of year that are apparently breeding seasons, of which there are a few a year, they jump and fight often and bang their heads on the tank cross-boards and walls so bad we can hear this in the house. I can't recall ever seeing any injury marks on the skull. Not that I looked for them, just can't say I've noticed.True,but there's usually other signs of injury in those cases.
That's why I said that they have to likely hit their head in the right spot and the head will look fine but not the eye anymore. Alf has been with us for 5 years and has gone through hundreds of fights and several successful courtships being the alpha male and only recently suddenly got the drop eye.