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Hornwort is good, as is watersprite, although they can both get meesy. Duckweed is liked by them for nest building, but most people do not like duckweed.
I agree. Frogbit and dwarf water lettuce are great too and not nearly as invasive as duckweed. On the other hand, there are researchers working to create bio-fuel from duckweed so you could have a future gas supply in your tank if you want.