First, you'll need to find the area of the base of the cylinder. Since the fish tank is a cylinder, its base is a circle, and you'll use \(\pi\)r^2.
r=(1/2)d = (1/2)30 = 15 inches
\(\pi\)15^2 = \(\pi\)225 (We'll worry about multiplying by pi later; for now, this is good enough).
Then, to find the volume, you just multiply by the height of the fish tank.
\(\pi\)22536 = \(\pi\)8100 = 25446.9005
The answer, then, is 25446.9005 cubic inches (or just 8100\(\pi\) cubic inches)