Oxygenating plants are an essential part of any aquatic ecosystem. They are particularly important in a garden pond, because they provide vast amounts of oxygen, which is not only important to the fish, but also wildlife. The underwater leaves absorb both minerals and carbon dioxide, helping to starve and therefore control algae.
It is mostly sold in bunches and must be planted into a basket using aquatic soil and topped with gravel. Don't just drop them into the water.