Anacharis (Elodea) are a type of "submerged" pond plant, which means that the entirety of the plant exists completely under water. The anacharis gets its own nutrients from fish waste, which helps clear up that type of contaminant left over in your pond.
Cleaning up this fish waste also helps rid your pond water of algae build-up, which uses the same type of material for the same purpose.
Water lettuce (Pistia stratiotes) is a type of plant that floats on the surface of your water with roots that dip down below into the soil. Water lettuce requires a pond that is not shaded in any way to properly grow.
Like anacharis plants, water lettuce uses the nutrients that would typically be used to spread algae for its own growing and healthy purposes.
Due to its deep roots, Water celery (Vallisneria Americana) is a type of plant referred to as a "filter" plant. The leaves and roots of water celery help to filter out certain bacteria and contaminants in your water that could at best make your water cloudy and a worst start to harm other types of plant life and even fish.
Water celery is available at most aquatic resource stores in your area and online.
Curly Pond Weed
Curly pond weed (Potamogeton crispus) is a type of plant that grows beneath the waters surface and helps maintain the overall oxygen level in your pond. It is officially known as an "oxygenation" plant for this reason.
The more balanced the oxygen level in your pool's water, the cleaner it will be and the harder it will be for bacteria that has the potential to negatively affect your pond to grow.