I can't grow duckweed?!

[mountaintopchicken]

This is so embarrassing to admit, but I can't grow duckweed. I'm actually a really good gardener and I can't grow one of the planets most noxious weeds. I have tried it in a stock tank and in a small plastic container, I have tried using organic liquid fish fertilizer and then Pete's professional powdered fertilizer. I even tried keeping it on top of a 5 gallon betta tank and it died - I think there was too much water movement. The duckweed never completely goes away in my non-fish tanks but never grows enough to feed the chickens.

What am I doing wrong? How is it possible not to be able to grow duckweed?

 

[Eris]

How deep was the water you were growing in on? Did you have artificial light?

 

[mountaintopchicken]

I used a 70 gallon stock tank and a plastic kitty litter container. Stock tank is at least 2 feet deep, kitty litter container more than 1 ft deep. They have been in natural sunlight outside except for during the winter. I overwintered the duckweed in half frozen state in my unheated sun room. It survived till spring but didn't grow, of course.