I have seen someone else keep altums with arowana in a planted tank. The altums are all adults, but they will crowd to the opposite side of the tank from where the arowana is. To me, they looked pretty stressed out.
At night, when the tank and room lights are off, when any of the altums reflect light from outside the window, the arowana will sometimes attack the fish, thinking it's a smaller fish.
On that note, I have to admit I used to have a 6 foot fully planted tank with arowana and I did keep other small fish with it, like SAE and Tiger Barbs. They didn't seem to be stressed by the arowana as they have lots of plants to hide in and can move about the tank safely near or in cover.
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