To fix this problem is simple...just put a fence around the tank. In Zoo Tycoon, each tile has a side that fence can be place on, so each side technically has the potential to hold two layers of fencing. The tank takes up one spot, so the new fence will take up the other. Make sure the fencing is something the tiger can't escape from, so it would probably be good to use the stone bar fence, like you have in place for the viewing area of the main land exhibit.
Hope this helps!
:original: