The stuff about fish growing only to the size of their tank, is total BS.
The reason most fish tend to gro0w slow in small tanks, is that water quality gets so bad, it effects the health of the fish and grows slowly not due to size, but to the deplorable environmental conditions, and bad health.
The only way you could possible keep a fish in a tank the size of puddle (and a 40 gallon is basically puddle) is to change 90% of the water every day.
Besides that, arowana come from the Amazon or other rives that offer them the space of millions of gallons, not a pot hole.