Do you have a UV filter? I had some really weird stuff show up in my tank after adding daphnia from a culture I bought that was obviously just collected from a nearby water source. Of course, you can always get weird stuff from purchased plants, anyway, but I won't ever add anything from a pond type source in my tank again. I ended up with some small fast worms on the glass that I'm pretty sure were planaria. Since I added cory cats, I haven't seen any on the glass, as the corys clean the glass up and down. I also ended up with mosquitos hatching in my apartment. I thought - How did that mosquito get in here? Haven't seen any outside. Then another one and another one - and I realized they were hatching in the daphnia tank lol. If I'd already had fish, they would have enjoyed eating the larvae. There were also some bloodworms, which are just a midge larvae. But, you can get some weird stuff show up - you don't know what's in that water.
A UV filter won't kill critters like planaria, but it will kill any bad bacteria pond plants or creatures bring into the tank, and won't hurt the good bacteria attached to the surfaces in the tank.
Sorry if I just freaked you out for no reason, if you've already put the duckweed in your tank
I'd just turn on that UV filter for awhile, in that case. It will take care of algae blooms, too. And just keep your eye out for anything bad like hydra or planaria, etc. I haven't seen the planaria bother anything in my tank yet, including the shrimp, so they may not be a parasitic type anyway. But, just keep your eye on your tank for weird stuff, is my point and I advise you to turn on a UV filter for awhile.
Ms. Walstad has suggested UV filters are great for killing bad stuff in the aquarium. I think they're really good preventive medicine. For what it's worth