ok, this might sound ridicules, but my duckweed refuses to grow! i wanted to start growing some for my goldfish, with no luck so far. i have some duckweed in an old concrete mixing tub, thats about 20/ 30 gallons? im not sure. at first i thought that the plants didnt have enough nutrients, so i added some old tank water. a week or so later i decided to add some soil to the water, because i heard its a great way to add nutrients. well, it did not work. the plants are alive but just, not growing? its been about a month since i first started this project. could i have some advise? at first i had a bubbler in the water, but i took it out because it was agitating the water too much.