Here's how to find how much VRAM is installed in your Mac:

Click on the Apple icon in the upper left corner of the display. Click on About this Mac. On the popup window that results, click More info...
The System Profiler will be displayed. In the left pane, there should be listed something like Graphics/Display (may vary depending upon your Mac's model and age). Click on that and you'll see your display specifications in the right pane. Your video memory will most likely be displayed as VRAM (total).

If it's less than 32 MB, you probably can't run Fish Tycoon.