From TX A&M Aquaplant

Common duckweed is a very small light green free-floating, seed bearing plant. Duckweed has 1 to 3 leaves, or fronds, of 1/16 to 1/8 inch in length. A single root (or root-hair) protrudes from each frond. Duckweeds tend to grow in dense colonies in quiet water, undisturbed by wave action. Often more than one species of duckweed will be associated together in these colonies. Duckweeds can be aggressive invaders of ponds and are often found mixed in with mosquito fern or watermeal. If colonies cover the surface of the water, then oxygen depletions and fish kills can occur. These plants should be controlled before they cover the entire surface of the pond.

That is a nice version above.

There is plenty here on DW. DW is the bane of fish ponds. In addition to causing an O2 depletion by covering the surface it kills the plankton (light denial) and thus the base of the food chain. The fish die from starvation or O2 depletion - take your choice. DW is very hard to kill.

http://www.pondboss.com/forums/ubbthreads.php?ubb=showflat&Number=35630&fpart=1

http://www.pondboss.com/forums/ubbthreads.php?ubb=showflat&Main=3551&Number=35724#Post35724

http://www.pondboss.com/forums/ubbthread...=true#Post34840