Sunshine; you can put just about anything in there that will put down roots. Impatients come to mind as well as going to the grocery store and buying some watercress. I hear it works well, is easy to pull when overgrown, and is prolific re rooting. And, should be easily available. I like iris, cardinal flower, pennywort, parrot's feather, and even have some hosta in mine. Also, Monkey flower is also prolific. I do have some creeping jenny in a portion of mine but it's more for decorative attributes than functional. Getting the plants to do your work is we're aiming for here.
And I'd probably put in a bit more gravel, just over the water line, to cut down on algae issues. Which btw, is another plant and IS helping you but not like plants with aggressive roots can.