I made the mistake of putting some in my small (1.5gallon) tank once. Current flow from my in-tank Whisper filter didn't matter. The only thing that slowed down the duckweed was lack of nitrates (and the subsequent BGA that showed up). If nitrates were up the surface would be covered (and thus current reduced) in less than a week.
72g bowfront planted, CO2, 2x - T5HO, Eheim 2213 and 2217, restarting after a move.
2g Mac-quarium. Clown gravel, fluorescent plastic plants, and 2 guppies.