Stucco had given me some duckweed awhile back. I eventually ended up feeding it all to the tilapia. So...I went on a hunt for some more. I found some, and brought home a 5 gallon bucket full. I noticed, this was much larger......so I did a little research......

Small duckweed.....Lemna valdiviana
http://plants.ifas.ufl.edu/node/226

Giant duckweed.....Lemna valdiviana
http://plants.ifas.ufl.edu/node/435

I've got the new stuff floating in a kiddie pool, with some bleach added. Hope that takes care of any 'extras' I may have picked up. Then I'll get to see if they'll eat the large stuff (?)
They eat about any leaf I've tried so far....