If your water feature is a one piece design and you know who manufactured it, you might find their website and see a recommended product that would be compatible to the material your waterfall is made of.
Using the wrong product could damage the surface of your feature and/or the pump itself. In the past, we have had waterfall birdbaths, outdoor zen water features and indoor water wall features. For those, we used an organic enzyme product that we purchased inexpensively online from Wild Birds Unlimited. However, it is best suited to small features (10 gallons or less) as the bottles are small and applied by drops each time water is added.
It may also help to adjust the location of the feature, if possible (sun vs shade) to help against algae re-growth, or at least to slow it down.
You may have to shut off and empty the feature, then scrub it down and begin anew. Much better results that way.
We currently have a large water feature in our yard and for this we use the puck-type chlorine made for swimming pools (as you can see in the photo) It has worked well in the year we have had it. We love it. No algae at all.
Good Luck and keep us posted!
