they are a cool fish, and they are worth more money the bigger they get, 6" isn't going to sell for much, so take it, grow it out in the 120. if you don't want to keep it, sell it. for people that keep arowana, if you lose a fish and or want another arowana they have to be close in size to keep together, even then its work to get them to adjust but supply and demand dictate that the larger arowana go for a good bit more money, least around here.
I really doubt it will eat any of your fish, less they have been raised on feeders, they prefer floating food and if you never feed live they shouldn't eat live. Bugs are preferred over fish, they are not that aggressive with other fish, least in my experience.
The only exception is top level fish. The only fish my arowana have ever eaten were other arowana. I've had 10 or so, and never had them eat any mid low level fish, and i feed floating pellets and frozen shrimp.
I have actually had one of my arowana fight a cichlid over a pellet that it could of easily swallowed, had no interest. Had another instance where a baby convict was dragging a shrimp into a hole and the arowana pulled the shrimp away from the convict and swam away. Idk your mileage may very. But only loss i've had to an arowana was when I put in a 4" with a 12", with in about 30 seconds the 12" swam to the button then shot up and ripped the 4" in half and he ate half and the half that sank was swallowed by a bichir.
Also on growth aggresstion, keeping your temperature on the lower end of the spectrum for the fish you keep will lower aggression and growth rate, raising has the opposite effect. I tend to keep everything at the highest temp that the fish in the tank can flourish. (83 in summer, 81 in winter have worked well for my stocking)
my main arowana was raised on feeders from previous owner, took a few weeks to get it on pellets, then shrimp and now i keep it with a few dozen fish it could swallow, has no interest.
My experience tells me Arowana only eat other fish when no other option is given.
