Azolla

A very small floating fern. Spreads quickly on still water. Will also grow on moist soil. Plants develop an iridescent reddish color in full sun. Forms such a dense covering on the surface of still water that it supposedly prevents mosquito larvae from developing underneath. (We have been using this plant to prevent mosquito larvae from living in containers where aquatic plants are grown, and it appears to be working well.) Often winterkills in cold areas, so it is best to keep a small amount indoors for the winter until you establish that it is hardy under your conditions. (Note: Possession of Azolla caroliniana may not be allowed in TX - residents of TX should check regulations.)