The video above is one of the best for both duckweed and string algae.
String Algae may begin to grow in your system. It's not all bad because it can be useful as a fish food, but it tends to clog the system. If your fish don't keep it under control a small amount of salt in the water will kill the string algae.
There are many varieties of duckweed. I have two, but one or the other will tend to prevail depending on the time of year. Duckweed can be rinsed and used in salads and it provides good protein for your fish. Under ideal conditions duckweed will double it's volume in just a few days, but you will probably have to raise your duckweed separate from your fish because they will eat it faster than it reproduces.
Duckweed cleans the water, but it can also exclude oxygen if it grows too thick. It prefers warm water, and seems to grow out of control when the conditions are right. Creating the right conditions can be difficult, so I spread it around various areas and let it grow where it wants to grow.
Good article about Duck Weed