I forgive duckweed of all it's defaults. It's such a fast grower, when I find there are too much. It is easy remove with a net, but I always keep some. Duckweed is not great with hang on filter, that filter tends to push the duckweed direction bottom, but does great with a canister.
Dwarf Lettuce is great, but the roots can become cumbersome. I guess it is a matter of taste :-)