Turns out I am quite happy with the duckweed. It continues to multiply, and so far is no trouble to maintain. Last week I noticed there were some pale, disintegrating leaves and I simply scooped those out when changing the water. Baby trumpet snails all move up to the surface film at dark and cluster in the duckweed floaters. I like seeing how Oliver swims up from below the scattered cover of plants, to look for his food offering.