No, I wasn't the one to discover duckweed to be a great indicator of Iron deficiency. Chuck Gadd told me about it. Since duckweed pulls all its nutrients from the water column it will tell us very quickly (2-3 days) if we lacks Iron in our water column by turning a red/brown. I use duckweed to help me measure the proper nutrient level in my tank. When I see mine turn red/brown I increase my fertilizer (I use TMG) 2 mL. I wait 2-3 days to see if the duckweed changes back to green. I keep increasing until I see the duckweed turn green. I do the opposite to make sure I am not adding more nutrients then my tank really needs. Once I get everything set and am happy I remove all the duckweed that I can. You can stick some rainbows in your tank and they will be happy to eat the left over.
Duckweed is a fast growing plant and should not be used in small tanks without monitoring it. If you add fertilizer to your tank everyday you should look and see if you need to remove some of your duckweed. Duckweed is sold throughout the US and should not be a problem to find at your LFS.