Hey Pond Girl,

The product Dan spoke of is a black waterfall foam. It is like the "Great Stuff" that you can purchase at Home Depot but it is black instead of yellow and it is fish safe.

If you build the waterfall you will need to build the shape of the falls and line it with an EPDM liner. "Do not use PVC" it will shrink a percentage each year.

Rule of thumb for the pump needed will be 1500 GPH per foot width of the waterfall. This flow will give you a good display and not a trickle of water. The foam I spoke of above is used to make sure the water does not leave on the sides of the waterfall and does not go under the rocks. You want to see the water on top not below the rocks. Let me know if you have any further questions. Have fun.