I keep mine in bare bottom. Used to keep a smaller arowana juv with gravels but it was a nightmare on cleaning. When the arowana strike the food the gravel got all stirred up and i can see tons of murky stuffs. Well yea i should have vacuum it but us arowanas keeper should know the risk of having the top open for even 1 second. So i prefer not to do gravel cleaning where i need to open the top and risk spooking these fellows.
In my own opinion arowanas are best kept in bare bottom or even a bare tank. as all arowana keeper will know when these fellow strike their food or get spooked its nothing small. they will dart around and anything like driftwood or decoration can risk hurting them. some might think their arowanas are calm and never jumps or spook but it's just like waiting for accidents to happen.
and there's also the risk of them swallowing the pebbles and gravel...
just went barebottom after 20+ years of gravel. what a relief! maintenance is a breeze now and really helped keep the trates low. aros were sometimes picking up food off the bottom and taking in rocks. never had a bowel issue, but ya never know.
I switched my Jar's tank to BB a few months ago, he didn't like it at first but quickly got used to it and I prefer it, esp over the stupid play sand I had prior.