I’m on the side of setting up s sump filter. Even a ten gallon tank can hold way more media than a fx6. The trick with a sump is plumbing the tank. But if you’re getting an arowana you need to take the time and do it right. Even if you don’t have the money for it all at once, buy it piece by piece. Eclipse overflow with the drill, run some flexible tubing down to a sump. Start off with a 10 or twenty gallon aquarium filled with some filter floss or a sock and then pack the rest with lava rock and a 500-800 gph pump running up to the tank.
From there you can upgrade as you get spare cash. Eshops makes some very affordable sumps, and aquacity has ceramic media for a fraction of the cost of biomaxx or something similar.
If you want to get the full set up all at once you’re looking at something like this.
Eclipse overflow-about $100
Eshops sump, less than 200-300 if I were to round up.
Aquacity media-$50
And then a pump and tubing for a return, you can spend hundreds on a pump or 50 dollars. So let’s say $100 for that.
You’re looking at maybe $400 for equipment you’ll have forever.
If you want to buy bulkheads and pvc and your own diamond hole saw you can cut out the eclipse and save about 70 dollars.
An arowana is a serious fish and it deserves a serious home.
If this doesn’t work for you, I would say get a fluval canister. Fx4 or the next smaller one. And then put a ac 110 on the back as well.
Sump is the way to go though. You can use it later for saltwater too if you want.