You can either (1) keep removing it until you finally get all of the little
leaves, or (3) put in some duckweed eaters. Buenos Aires Tetras will chow
down on the stuff like crazy, but they also will eat EVERYTHING GREEN in your
tank. They MIGHT be OK, but I would certainly watch them closely. Common
Goldfish really go for duckweed, and are not so damaging to the other plants.
I really recommend the small net on a daily duckweed removal task until it is
all gone. It is safer, even though slower.
Hope that helps.
ps. Back quite a while I offered starts of a "Giant Duckweed" I have which is
a lot easier to control. I think it may be Lemna gibba, but the various
sources disagree. I did not send it out to those who asked for it, because I
could not figure out how! List Member Dirk picked up some personally
during the Iowa State Fair, and he suggested I could probably send it just by
ordinary mail, in zip-lock bags in envelopes. So, if anyone is still
interested, you can email me again, and this time I will try the mails to
send it. No charge, folks.
If Dirk is reading this, I wonder? Dirk, how did the stuff make the trip in
the zip-lock bag?