i was under the impression that a fish with drop eye (from developing it in a tank) can often be cured by placing it in a pond. maybe the fish seen in the pond picture with drop eye were put there to fix it or had an injury. i dont think its "poor" genetics but something we can expect from arows by placing them in an environment that they have not evolved to suit. so in this, the genetic of arows are what they are, and we give them a tank that does not suit them so well.
if you look out toward a tv a long way across the room and then force yourself to quickly look down at your lap and toward your neck and focus fast and do that again and again you will get eye strain and might even be able to feel the extremity that your eye can go to and feel a little pain.
so i think the action the fish does is at its extreme point of what its eye will allow.
to do so means that the top muscle needs to stretch out and the bottom muscle to pull in tight, over time you might get some sag happening. some arows evolved in different habitats will have ability to make this movement without as much strain.
to see if this theory holds weight we could check out the morphology work the scientists did in the paper on the different varieties, then also check out the eye position on leis and compare that to jars. the leis seem to have an eye position more suited to taking insects and may explain why leis seem to get it more than jars.
it might be enlightening to know what form of asians get drop eye the most and then check the eye positioning and feeding habits compared to the ones that are least vulnerable. anyone have any ideas on which colour forms get it more so?
the other possibility is that some decent percentage is from an injury and maybe even from hitting the glass wall. lots of theories and a few of them or all of them might hold true for certain individuals. im not gonna be one eyed on it!
if you think about how both human eyes merge what each one sees into one picture..and fish have eyes either side of the head.. so they would probably be set up to merge the overhead or forward veiwpoint together.