There are various methods you can try besides the pin pong ball one. Try covering all sides of the tank except the top so that the fish is forced to look up and lower the water volume to about half. You can also place blinking bicycle lights at different positions on top of the tank every night or one of those LED lights so that the wro will look up. You can also get a shallow fibre glass pond for the arowana and leave it outside so that it kinda resembles a natural habitat. There are operations that can be performed to remove the fat cells behind the eye so that it no longer droops but it isnt permanant and will get droopy eye again from what i hear.