OK, so like I said - duckweed won't be ideal since it will just get sucked up and/or clog the intake sieve.
I have no clue about your lights - but if they are bright, you could go with something sturdier such as water hyacinth, which has gorgeous flowers, or water lettuce (lovely fuzzy leaves.) Both have large fibrous root systems which are excellent cover, and will help clean the water. That brings me to my next question: What size tank ?
The problem with this kind of thing is; plants do constantly clog filters - either the roots, or the plants themselves, or pieces of plants that fall apart - you will end up having a lot more maintenance and have to clear the intakes every day. That's the problem with floating plants.
You would do much better with plants in the substrate - just get ones that will eventually get big / tall enough to have a mass near the surface - such as Anacharis or Foxtail.