What does duck weed look like?

Petsolutions.com sells the stuff, if you look for it there, you'll find a pic (greatly magnified)

When I had duckweed in my main tank, it would grow so thick that it covered the entire surface of the tank in a week. I would scoop out cups of the stuff each water change. When I started supplementing CO2, the underwater plants took off and the duckweed died off--completely. I still have a few bits in other tanks with no CO2, but nothing as bad as in my 40. It really depends on the available nutrients.

There are a couple of floating plants like duckweed--frogbit, and one I can't recall right now. Salvinis? Maybe...Anyway, they don't get as out of control as duckweed, even when nutrients are plentiful, IME.

Salvinia natans? I have it in my low-light tank and it grows enough to need thinning out weekly. It grows better with more light but looks good IMO anyway.

here's a link to a picture.

Ah! Yep, that's the stuff. Thanks SBA--This week I am working with a reptiles and amphibians document, and all that came to mind was Rana pipiens...Not the same thing at all.