Earlier this summer we had a terrible case of algae in our academic quad. We tried everything (almost) including filters, chemicals, added pumps to move the water around, just nothing worked. Then we bought some pond plants and that cleared the water within one week. It's amazing and, the pond lettuce plants have had babies and babies and babies, so many babies that the surface of the water is almost totally covered. Started with 8 pond lettuces and now too many small babies to count.
Before with just a few plants on the upper right
The only green in the water is the plants.