Submerged Plants. also called Oxygenators, grow with their roots anchored in soil, but the leaves stay underwater. Oxygenators are essential for keeping the pond healthy and the water clear. The best known examples include Anacharis (a deep green plant with many delicate leaves, which will grow in water 15cm to 150cm deep) and Hornwort (dark green grass-like leaves, need 15cm to 30cm of water above the crown).
These plants absorb carbon dioxide and release oxygen into the water, and by oxygenating the water they help it support more aquatic life such as fish and beneficial aquatic insects. They also absorb excess nutrients in the water and help purify it, which reduces the growth of algae. Oxygenators also provide food and shelter for fish. The leaves, which are usually fern-like, lacy, or hairy, provide cover for the microscopic aquatic life forms which are an essential part of a balanced aquatic ecosystem. From here.