What your describing sounds like Parrots Feather, Anacharis/elodea, Cabomba or hornwort. They are all invasive.

Sky, I grow mine in a pot with no drainage. I leave at least an inch at the top of no soil so I keep it soaking wet. That way I can have it in a inch of water. I also give it 1 tbl of epsom salt in a gallon of water once a month. I have it in morning sun. Which I am sure is not real sun compared to your zone. In your zone I would probably keep it in the shade. If you do these things I think your browning will stop. I have also grown Pink Sunburst and Tropicana in bog condition. The hotter it is the more water it seems cannas can take. If you doubt this wet process for this plant check out Bernie here in the Canna forum. http://forums.gardenweb.com/forums/load/canna/msg0814564325672.html

Hey, I can trade you some more colors of irises for the Pink oriental lily. If not interested in that, I have some other bulbs too, Cannas, and a small Bamboo for the pond. Maybe a small water lily. Probably some other stuff too. I have a small pond similar to yours. where do you live in Upstate SC?

Any particular plants that you are interested in? You wouldn't happen to have Pinecone Ginger would you? I'm interested in the peach brumansia and the amaryllis. Also the red canna, the pink lily (#19), and some bromiliads. I have Blue Flag Iris, Giant Pennywort, Parrot Feather, Frogsbit, and maybe a Yellow Floating Heart (it might be small) If there is something else that you might be looking for just ask, I might still have it.