Most people say that duckweed is the worst and that you can never get rid of it, but no matter how hard I try, I'm never able to keep it alive for more than two weeks!!!!
I have duckweed...and a fastish flow in my 53 gallon.
The duckweed does not last long but not for the usual reason
If the duckweed dares to venture into the flow of either filter, (which is a big misjudgement on the part of the duckweed), the filters will fling the (either very brave or very stupid) duckweed into a stream of high speed somersaults that a gymnast would be proud of
Unfortunately during this gymnastic display by the duckweed, it sometimes slaps a fish or two around the face (something like slapping a human face with wet lettuce)
The fish being slapped by the soggy and dazed duckweed are generally none too impressed by that behaviour, so they chase the duckweed down.....
.........and eat it.