Duckweed-Click for more info

The families of duckweeds are the smallest flowering plants.  These plants grow floating in still or slow-moving fresh water across the world, except in the coldest regions.  The growth of these high-protein plants can be extremely rapid.  University of Florida, Center for Aquatic and Invasive Plants.

Individual duckweed plants are about an 1/8^th to a ¼ of an inch in width. They are generally round in outline, with a single root hanging from each small plant. 

Carole A. Lembi, Professor of Botany
Botany and Plant Pathology, Purdue University

Duckweed is one of the most common weed problems to the pond owner.
Fortunately, Duckweed is very easy to control.  

Ponds Rx recommends using a Sonar product for controlling duckweed species.  There are two choices of formulations available to choose from-pellets and liquid.  We recommend Sonar because of its long success record in controlling nuisance aquatic weed problems without causing bad side effects.  It will take between 30 and 45 days in most cases to eliminate the duckweed from your pond.  Although this seems like a long time, this is good for the health of your pond.  The oxygen level should not drop to a stressful level for the fish.  The Duckweed will first turn white or pink.  This should happen within the first 14 days.  After this, the Duckweed will slowly drop off of the surface.
Toxic to weeds, but...Safe for fish, cows, dogs, swimming, fishing, and of course, frogs and turtles!!