We bought some duckweed with our first order of plants. I put it into a few tanks and decided I didn't like it. Removed it from all of them. A few months later it exploded in one tank - no idea where it came from since i did get it all out, I thought. Now it's in every tank in the house.
I have a bunch of various floaters - duckweed, greater duckweed (beautiful and better than small duckweed), frogbit, salvinia, antler fern and 1 piece of dwarf water lettuce that I hope spreads. With the exception of the small duckweed I really love all the floaters.