DuckWeed out of nowhere!!!

Last week when I uncovered the tank I got a SHOCK! 8-()
The surface was covered totally with Duckweed!!! I wondered why the tank looked like a Low-light set-up (for a minute I thought some of the lamps where dead)
This weed developed after just one week. Not that long ago I added one Lotus Zenkeri and probably one tiny Duckweed slipped into the tank. Enough to take over the whole aquarium!
I took a fish net and removed all the Duckweed. There is probably some left.