Submerged water plants can often times be a great asset to a pond as they help with water quility as well as clarity. Submerged plants help filter unwanted nutrients and add important oxygen to the water durning the day. Underwater plants are referred to as "oxygenators" because they add oxygen to the pond (only durning the day). At night these plants reverse the process and remove oxygen from the water. Submerged plants vary greatly in their foliage and flower habits, some have both submerged and emerged foliage, some have only submerged leaves with floating flowers, and then there are some with both their leaves and flowers fully submerged. Submerged plants grow in a wide range of water conditions, from sun exposure to water depth. Vallisneria Americana and Vallisneria Giigantea are two popular submerged plants.