I'm not entirely sure I buy the whole "looking down" theory. I mean, how much does an aro need to look down before it develops DE? Even if there are bottom dwellers in the tank, it wouldn't mean that the aro would be looking down 24/7. Something I've observed recently about aro eyesight. When I throw mealworms in and they float, my aro will see them and eat them. If they start to sink, he will still see them if they are in the 90 deg angle between the level of his eye and the top of the water where his head is. Once it drops a further 5 deg lower than his eye level, he doesn't see it anymore and doesn't chase it. Even if he is far away enough to see the mealworm dropping, he will pursue it, but once it drops below the level of vision, he turns around as though nothing happened.
This is reasonable, since aros are top/ middle feeders, that's why their eyes are designed like that. Lots of fish keepers in Asia keep aros with stingrays with no DE. I suspect that at the end of the day, it's a low-fat diet with good genes that will guarantee no DE.