Water lettuce and hyancinth are excellent nitrate removers. They are considered invasive in some locations. I use them in my pond, they reproduce very quickly and outcompete algae. As the lettuce and hyacinth reproduce, I throw them in the compost bin. They make great fertilizer.
Water lilies are also good in ponds as they will shade the water. Bog plants such as cattails, arum, canna, lizards tail, and lotus are also good.