We have water hyacinth and water lillies.

Hyacinth are $3.50 each plant or 5 for $15.

These plants float on the surface of the pond, giving your koi a place to get out of the sun, and also reducing the amount of sunlight that reaches into your pond, discouraging nuisance plant growth.  Their hairy root system sucks nutrients out of the water as well, which discourages algae growth.  Some folks also like to put them in their water falls (you can hold them back from flowing down with a little bit of fishing line).  They like flowing water, but they don't like the cold....they will not survive a Wisconsin winter.

Bonus:  pretty flowers!

Water Lillies: price varies, starting at $8.50 per plant.