Submerged Oxygenating plants can be hardy or annual. The true aquatics (Hornwort, Vallisneria, and Anacharis) can grow very deep, sometimes more than 6-8' once established (best planted 2' deep). The emergent varieties do best in 2 feet or less of water and can even grow a few inches above the water (Rotala, Ludwigia, Moneywort, Mermaid). Hornwort is by far the best oxygenator. It is winter hardy, emits natural allelopathic chemicals that retard some algae growth, and is native/legal in most states.