Well.... there's true duckweed that is a small shiny smooth leafed floating
plant that goldfish and Koi love.

Um:

http://www.mobot.org/jwcross/duckweed/duckweed.htm

This website will tell you why I asked /which/ duckweed.

Although I can appreciate that perhaps rec.ponds'ers maybe
aren't as knowledgeable about duckweed as all that. You'd
practically have to be a botanist.

C//