I know someone who kept altums with arowana. The arowana did not bother the altums in the day, when it could see that the altums were too big to swallow. However, night time is a different story. When the room is dark, and the altums' silvery scales reflected light, the arowana would attack.
I also know someone who had 2 feeder goldfish together with his arowana for months at least. For some reason the arowana refuse to eat them, although they were small enough to be eat them. They were removed later because their excess waste was causing algae problems.
I kept tiger barbs, otos, malayan shrimp, yamato shrimps and SAEs with my arowana in a planted tank. The shrimps survive. The barbs and SAEs were too agile and careful for the arowana and had plenty of space to hide. Neverthless, at least one SAE was eaten, albeit accidentally. (That was before SAEs was in his menu. The SAE was nibbling on his frozen prawns when he made a sudden turn to snatch it back. The SAE was eaten in together with the shrimp.) Otos he ignored... I suspect he tried one once, but the pointy armor taught him a lesson he never forgot. He was lucky he could spit it out... else it might lodge in the throat and killed/starved him.
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