Aquathol Super K @ 3 ppm. Apply around the perimeter on the weeds and allow the product (active ingredient endothall) to equilibrate across the pond. This will extend the exposure time and aid with killing the milfoil. I'd go ahead and treat to prevent the plant from putting down any more seed this season. Based on your calculations you'd need about 13 lbs of product. New packages are shipping and come in 20 lb bags. You don't have enough coverage of weeds in the pond to be worried about DO issues.
