Frogbit (Limnobium spongia) Clusters of heart-shaped leaves float on the water surface. This lovely little floating plant looks like a tiny waterlily.  It has small shaped leaves that are produced in neat rosettes and tiny white petalled flowers with yellow centers. Grows in sun to part shade with a running spread, reaching about 1/2 inch tall.  Hardy in Zones 7-10.  You can divide this plant during the growing season. 

Another plant that will float on your water surface creating shade for your fish and helping with water clarity.