Can't grow duckweed

This is not a joke. I want to grow some duckweed in my 10 gallon tank so I can add it to my goldfish setup but it's been 6 months and this is the maximum amount of duckweed that I can retain.

Forget about growing them I'm worried that it might go extinct in my tank. Both underwater and over the water line the other plants seem to be doing okay but I can't get duckweed to grow for some reason. I tried feeding the tank 4 times more but it's still not working. I don't use fertilizer but I tried adding iron and magnesium supplements but that also didn't work.