But, you should watch your fish and see how much your arowana eats each time. If he's fat the next time you go to feed him, then you don't need to give him any more. But generally, like quoted, 2-3 times a day for small guys and you only need to feed them once a day when they're bigger.
Silver have an excellerated growth rate up to 24", so I fed mine daily up to about 22". Alot will also depend on what you are feeding it. I would just avoid stuffing the aro, and feed smaller portions daily at the size it is now.