What are the dimensions? If the tank is truly 6'×30" I wouldn't say the conditions are horrible. If the rays are much smaller than the aro I could see a problem.
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200g is little when it comes to a 30 inch fish! Bottom line, end of story. There is nothing anyone else can tell you that will be helpful. You have two choices to solve your dilemma, buy a bigger tank or sell the aro.
Better question would be why is an arowana attacking a ray or other fish?
Definitely an underlying reason for why it is happening. Arowana are nice friendly fish but they are space hogs(meaning they need allot of room) and if the area is to small they act out by attacking other fish. At least from my experience.
If your tank is 72x30x20 the rays are getting in the arowana bubble being such a low tank and thus the reason for the attacks.