Our rocks get green algae!

by Tcooper9
We have a 20 ft long waterfall feature that we leave running all summer long (Upstate NY). The rocks in the river bed and the rocks in the pond at the bottom keep turning green with algae. I spray them down with the hose nozzle every couple of days and have also put pieces if chlorine tablets in. The water is clear but the algae never goes away. Ideas?
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