My plan is to build a 370-500 gallon aquarium or how ever big it turns out out of plywood with this 220 gallon within the next year. Of course I know I have to seal the wood and all that. I saw how The King of DIY did it for his Arowana and that is the same plan I have.
I would go bigger then a 500 for 2 full grown silver arrows especially if you plan on putting other fish in there...
Aim for a 8x4x2.5 min
Trust me on this