so in one of my tanks i have something that acts and looks like duckweed...... and no this is not a case of if it looks like a duck and sounds like a duck its duckweed lol. the difference is the bottom of the leaves gets red/purple and the roots tend to have several off the same plant not like duckweed that has a single root in one place. these are the best pics i could get right now hope it helps.
top of the plant
side/root shot... yes its covering the back wall thanks to evap in the tank lol
underside with red/purple color