You could try duckweed, water lettuce, and frogbit. That's the most floating plants I can think of at the moment lol. Duckweed RAPIDLY multiplies, and for reference, I added a small handful to the surface of my 60 gallon, and within 5 days the whole surface was covered. I also have dwarf water lettuce and duckweed in my tank to provide shade for my low light plants since I have such high light. But you have to keep pulling some out almost every day to keep it under control. I also run a bunch of bubblers and a powerful water circulator (lots of surface movement) and they do just fine.