As long as you aint stocked it yet id use gly and a surfactant, ive used copper sulfate to kill underwater aquatic weeds but ive never sprayed it on lilly pads. If you got fish in it id be very VERY cautious about how much chemical id use if any at all. Id probly make me a re-bar drag and dragem out with a chain and 4wheeler,tractor, or truck. I do know gly will kill lilly pads but im not a 100% on the copper. I do know that the copper sulfate is safe for fish as long as you go by the lable...if you dont and you spray to much at once the bacteria from the dying weeds will suck up all the oxygen and turn the pond over...ask me how i know. Be careful if you got any fish in it..if it hasnt been stocked yet id make sure it was as clean as posdible before i put any in there. But i do know that dragging is about the safest method when workin around fish.