I would like to add that duckweed, although very easy to grow and find anywhere, is a terrible mistake,. Especialy if you have any kind of mechanical filtration from a simple HoB to canister filters. Duck weed is an insatiable plant that will suck all the nutrients from the water and cover every above water surface imaginable. It will also deprive the water of oxygen if you don't have any air stones/ air system set up. This stuff gets into your filters, so even if you want to scoop it all out it can AND WILL come back. The only way to get rid of the stuff for good is to take down the entire tank and let everything dry out.
Secondly, disposing of it during regular pruning is risky. If the plant gets into any local water supply or ponds/lakes/bodies of water it can take over and wreak havoc on the ecosystem. All in all this is a pesky plant.