Water Hyacinth (Eichhornia sp.)
Where it's found in nature: Tropical and sub-tropical South America.
This floating plant is tropical which means you will have to protect it from frost. It will get a purple flower towards the end of summer. It multiplies quickly. It's roots that go about 3" down in the water take out nutrients from the water that would otherwise feed algae growth. This plant likes full sun and will not do as well in a shaded pond.