I am a firm believer that drop eye is caused by diet more than anything else. Many people think that the reflection on the base of the tank causes them to look down. This is not true as the arowana does not see this reflection due to refraction in the water; it is only yourself that will see the reflection because you are not in the water.
To back up my theory, as you can see in the below video fatty deposits are being removed from behind the arowana's eye sockets caused by a bad diet of probably mamallian meat.
http://www.youtube.com/watch?v=V2LEclFiZAA&feature=related