a good friend of mine had 2 silver arowanas in a 125 gallon tank. in the tank they had drop eye. he has since moved them into a 750 gallon indoor pond and they have lost the drop eye. the cause of the drop eye may be as simple as the arowana looking down all the time because there is stuff to look at. in the pond there is no glass to see out of - only up - so they are constantly looking up rather than down or out. remember, in the wild they eat things out of trees so they are constantly looking up into the branches. i have seen people put a ping pong ball on top of the waters so the arowana looks up at it (just make sure your arowana cant swallow it because it will die. mine died from eating a suction cup). other causes of drop eye are fat build up behind the eyes due to fatty foods such as goldfish. try getting your arowana on a diet of hikari pellets. they are healthy and the arowana will have to look up to eat them.
hope that helps
check out my video of my arowana eating pellets
http://www.youtube.com/watch?v=DLgGGwxy5wI
he only had drop eye in one of his eyes (weird) but i didnt really do anything to try to fix it because it wasnt that bad in my opinion.