An Arowana can get droop eye syndrome when its eyeball is continuing tilted downwards such that it is always looking down. This usually occurs in only 1 eye, with the other eye looking normal. The severity of droop eye may vary from a slight tilt to an extreme case where a large part of the top of the eye ball is exposed.
The following video shows an extreme case of this.
Causes and remedies of droop eye
Diet: I have been told that high fat diet causes fatty deposits to be found behind the eye socket. Perhaps autopsies were done on specimens with droop eye and these were the findings.
Another diet factor that has been mentioned is the feeding of hard or heavy food. This theory comes along because it was observed that the Arowana appears to be looking down when eating.
Genetics: Genetics could be the cause but its very difficult to prove it right or wrong. But whatever it is it cannot be proven wrong may more or less be presumed right until proven otherwise.
Another genetic possibility could be the varying degree of curiousness of each individual fish. Just like us, each individual fish has its temperament and character also. Perhaps individual Arowanas that are more inquisitive may be more prone to droop eye.
Environment: Environmental circumstances have been attributed to cause droop eye in Arowanas. Among these circumstances are the reflection on the tank bottom and bright lights at the top of the tank.
Another often quoted possible cause for drop eye is the customary looking down by the Arowana. Arowanas being very intelligent fishes, as intelligent as a fish can get I suppose, are also inquisitive as well. As the fishes are placed in tanks higher up, they also have a tendency to look down on moving objects much related to seeing out of the tank.