Just sharing some of my experience with y'all... About a week ago, I bought a bag of floating duckweed from the aquarium society auction. I thought, this will be nice... some cover for my tank, maybe a little snack for my goldies, this will be great. Wrong! I put about a pound of these little leaves in my tank, and two days later ALL of it was gone! There was a lot of green goldie poo on the tank botton and my fish were all looking at me like... "We didn't do it! maybe we did... but, but mom... we got hungry, and I was only gonna eat one, but the other guys started egging me on and... well..."

It actually did work,you had cover for a day or two,and the Goldies snacked like pigs!
I've gone this route before too,and had the same experience.Those guys love that stuff.

I'd definetely suggest to grow duckweed, and most smaller floating plants, like Azolla, somewhere else outside a hungry goldfish's mouth. A separate smaller aquarium or bucket would be a good solution, preferably outside in the sun. Otherwise - salad buffet, here I come!