Can I cast a vote for good old watersprite? It may be the easiest of the floaters to prune and keep under control. The bigger plants don’t mind a trimming of a couple of ranger leaves at all and the new plantlets are a snap compared to duckweed. Over the years I’ve had no luck whatsoever with the other neat floaters like azolla, red roots, etc. You may also need to consider what plants you have under whatever you choose.