duckweed

This might sound a bit unrealistic - but I'm looking for an easy grow floating plant that also does not block out light as I've got some wisterias, bacopa C, vesicularis/Xmas moss and egaria densa growing from the substrate. Its a low-tech tank w/ moderate lighting and only on liquid ferts...

So I've tried to keep floating plants in my covered betta tanks (on two continents!) and always after explosive growth the populations collapse. Makes me feel like an idiot because, well, it's freaking duckweed/water lettuce. This time I pulled some duckweed and water lettuce aside and put...