I believe you meant:
0.5 fish
------------
ft^3
instead of:
The density of a fish tank is 0.5 \frac{fish}{feet} ³. fish^3 has no meaning. ;)
Try this:
30 fish
-------------------- = 60 ft^3. If the tank were a cube, it'd have side length 3.9 feet.
0.5 fish / ft^3
