Duckweed can take over a tank pretty quickly. Depending on what you are after that may be good or bad. It is easy to end up with lots from a couple of bits hitchhiking home from the lfs
with other plants. If you have only low light plants, you may like the cover, but remember that it will block out a lot of light. I will admit that I find it to be a nuisance,.
An easy viney-type plant is hornwort, which will also take the place over if given the chance. I have done this on purpose with great results! Plus, its basically bulletproof.