In a simple sentence: Yes, if they fit in its mouth.
Do not put any small fishes in the same tank that you are not prepared to lose. The arowana is a predatory fish after all.
However, in my experience, it is a rare occurrence. My current arowana has taught me that it differs from fish to fish. I have never seen my previous arowana actively hunt down its tank mates. However, my current one, when it is active, will try to eat the SAEs. The SAEs have so far been able to stay away or escape.
- A lush planted tank provides lots of cover and the prey are smart enough to stay hidden when the arowana is active.
- The same smart prey knows how to stay away from the jaws of death. I had Siamese Algae Eaters and Tiger Barbs in the tank with my previous arowana and sometimes they still dare to swim in the open, but only behind the arowana. Once it turns around, they go back hiding in the plants. My current batch of SAEs stay near or under cover mostly, they only swim in open water when they smell the arowana’s pellets, to try a grab any crumbs or missed pellets.
- Most predators will not expend unnecessary energy to catch its food. If your arowana gets fed regularly and fed well enough each time, it should wait for the easy food and not bother to chase difficult food. However,my current arowana is proving me wrong.
- If possible, get an arowana that was trained from young to eat non-live food. Like pellets and market prawns.
- Keep middle and bottom swimmers. Arowanas have upturned mouths which are designed for prey swimming near or on the surface, and hanging off plants above water. An aggressive arowana may still try to eat the such tank mates but is not well equipped to do so.
- Keep fishes that are agile and fast.
- Do not keep too many other fishes.
- Do not keep any fish or other tank makes that have spikes, thorns, poison, etc. E.g. Otocinclus and snails with spiky shells (I have seen my first arowana chomping on tiny snails a couple or times).
Fish that are too large to be eaten are safe from the arowana. However, you may want to consider whether your arowana is safe from them. Also, a planted tank needs to maintain comparatively low levels of Ammonium/Ammonia, Nitrites and Nitrates. Having a high fish load would make it difficult to keep the algae away.