duckweed eradicator

I found that after a week away from home (and w/o the daily feedings), my
angels had developed a taste for duckweed.  Two of them kept what was a very
healthy growth down to very little in a 29H tank, if I just did a day or two of
missed feedings every week after that (which I've read is healthier for them
anyway--and, sure enough, they continued to spawn, so it didn't seem a