Take out the billions and make the problem simpler, because it's all a matter of proportion.
A 3-inch long goldfish needs 3*3=9 gallons of water. A billion goldfish need 9 billion (US) gallons.
An aquarium holds 6.3 gallons of water, which is not enough for one goldfish. In fact, it's only enough for 6.3/9=0.7 goldfish.
So the Georgia Aquarium can hold 0.7 billion goldfish=700,000,000 (700 million).