I have a duckweed FIX. It consists of two words
Yup, I have a small 2 gallon SV and taped a smaller 3/4" filter hose to the intake, and it SLURRRPPSSS that duckweed right up! It takes me a couple of "visits" to the tank to get it all, but I have successfully wiped it out in two tanks. It gets those annoying pests hanging out in the corners, it just cannot hide!
Just be careful with those top dwelling fish, I have to chase my endlers from the intake, silly fish.