Question
This is the first season  for pondless waterfall and the rocks in the "strean" are BLACK.  I also have some string alge.  What do I use to bring the rocks in the stream back to norman color?  The way the stream bed looks is not very pretty.  The stream is bordered with various vegatation and feature stone lanterns.  Can you help?

Thanks

Arlene

Answer
In order to help you better I would need some more information:
-Do you have any kind of filtering system on your waterfall?. How much water is there? Would you be able to send me pictures?

There are several factors that would turn your rocks black.
It could be the water/plant ratio, high nitrogen content,  poor filtering system or a combination of all. Those are the things that will promote the growth of algae on your stones, and they will turn them black.

High nitrogen contents on bought plants and soil is usually the cause.

Try to scrub all the stones and change the water. Avoid using chemical fertilizers at all.

In Europe, many people are doing swimming ponds instead of swimming pools. They are beautiful water features that could be used as swimming pools. They are maintenance free and all the filtering system is done by proper design and plants. The water is crystal clear and is usually considered suitable for drinking. Just what nature does.
The initial cost is a bit higher than a regular swimming pool, but the savings on maintenance and chemicals will make it worth.

Hoping to hear from you

Carlo