You'll be able to ID them better with web pics
There are one group that have clusters of leafs, and another that have individual leafs
Plus the size is a factor
Tiny stuff, like HC size, or Giant, with leafs bigger than the size of a pencil eraser
Oh, there's also soft leafs, and rubbery leafs
Sorry I can't give you an actual ID
, but I'll say it's definately a duckweed of some kind
It's been awhile, so have a look
here for some good ID help