Simple. The tank is 72 gallons, 16632^3. Just take 72 and multiply by 231.
The non-bow front part of the tank is 12x48x18, right? Thats 44 gallons. That means the bow part is 28 gallons. The length of it is 48" and the height is 18" that means the bow is 7.45" deep.
Thats 10368 for the non bow part, and another 6436 for the bow.
Comes out to ~16800.