The above image shows, Water Lettuce (Pistia stratiotes) in my tank. This is a free-floating plant with little thick and spongy spongy, pale green leaves. The leaves are covered with tiny hairs, making them water-resistant. Usually the leaves are 1 to 6 inches wide in wild, but in indoor setups, it might be much smaller. This is a very aggressive invader and can grow very very fast. This has no known direct food value to wildlife and is considered a pest species.
Pistia could be used very effectively in new setups, to absorb extra nutirents that couses algae bloom, but it needs to be monitored very closely on a regular basis, so that it does not cover the whole tank.
Pistia shows a very interresting instance of Allelopathy (Allelopathy - a chemical process that a plant uses to keep other plants from growing too close to it. See Wiki ) with Salvina natans. If Salvina is introduced to a tank that is infested with Pistia, within a week or so, all pistia will disappear and melt away, leaving only the Salvina in the tank.