40 gallons? Hope you mean for a baby, maybe you should specify.
Properly cared for, Silvers grow fast 1-1.5" (with growth spurts up to 3" per month)and don't slow down until 24-25" range. So figure within 18 months you'll have a 2 foot monster on your hands, maybe even sooner if ya start with a larger juvie.
Your aro should be keep in a tank that is at least 3 times it's length and have a width/depth at least equal to it's length. I.E. a 180g 6'x2'x2' tank will be good for a silver up to around the 24" range.
Diet, you can feed a wide range of foods, but it is best to get your aro eating a quality pellet food first. Once it's eating pellets you can expand it's diet. If the aro is a baby, under 3", best food to start it one is freeze dried blood worms. Then expanding to pellets at about 4".
Have a read of this; lots of good info there.
http://www.monsterfishkeepers.com/f...9584-do-you-want-an-arowana-...then-read-this