Animation of Water in a Tank

Hey Everybody,

How can I animate a reservoir with a decreasing fluid level?  I thought that I could just create an assembly with a reservoir and a family table of three different heights of fluid.  I then thought that I might be able to turn the tallest "fluid" off and the "medium" height fluid on....and then turn off the "medium" and then turn on the "low".

But I cannot figure out how to incorporate HIDE/UNHIDE into the animation.  Any suggestions?

I would like the animation to be very primitive.  I am not interested in CFD for this animation.  Just a basic animation of decreasing fluid level.

TIA,

Neal