Algodoo

Balls are created in the hollow purple box, and are
liquified if they contact the tube.
In the lever is a script which sets variable scene.my.lever to true
or false, which turns the pond on or off.
You can delete the red balls, they are only for
demonstration that red balls are not liquified.
If you want to enhance the flow of water, you can
enter the script of the solid purple box, where
you can reduce the number 20 to any smaller number
greater then 1, then turn of and on again to restart
the pond.