My roseline sharks that I had occasionally fed on the duckweed. Probably another reason it faded away, lol. Also, with regular PWC's, duckweed won't last, as the nitrites are not high enough to sustain them. If you want to keep duckweed and have tons, let your nitrates increase a bit. But don't let it get too high for the fish. My duckweed problem I had was when I had 30-40ppm nitrates. Now my nitrates are low, as I have to dose them to keep the plants healthy, and the duckweed is gone.