Sounds like a broken back. Tbh not much you can do. If it still acts healthy and is eating it may not be an issue. Just have to keep an eye on it. What size tank and tankmates?
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Stress won't cause a broken back. The larger tinfoil barb may have spooked him. Young aarows are very skittish. He probably slammed into the side of the tank.
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When i had my silver arowana I used to think he had never jumped either until one day I stayed by the tank with all lights off and sure enough every once in a while you can see or hear them jump. It dosent take mutch for them to break their backs, the are a very powerful fish.
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