I have some beastly ones that are quite dangerous. One could mistake them for lobsters. Those of course live in the pond, not in any tanks I would keep anything else in.
As for duckweed, I hope you are in for a serious commitment in terms of scooping out handfuls of duckweed every so often. As for me, I find it much, much easier than trimming most plants. As long as you don't let the duckweed get to the point where it chokes light, you'll be fine.
Aesthetically, I love the way it grows and bunches on the surface. It's also been known to be a tasty snack for some of my fish. you'd do well to have this in an ACF tank! Although you should consider semi terrestrial frogs and lilypads instead!
I was thinking amazon frogbit, but I am still trying to locate some. It has a nice look to it.