Even though they are illegal to have here in Florida, I use water hyacynth (sp). They are extremely effective in keeping the water clean, plus they have pretty lavender flowers. I collect them from the wild (so I am actually helping the state rid the public waters of this extremely invasive plant) and put them in my little pond. They reproduce rapidly so once the pond gets too crowded, I remove the older, rattier looking ones, seal them in a trash bag and toss them in the garbage. I've never had to add any chemicals to my little pond as long as I have had this plant in there.
