Tom :

Drive a post into the pond bottom where you want the tree and tie the tree to it with strong nylon trot line string ( I use black). It will soon water log and stay put. If you think it might get loose in addition add the concrete as ML suggested. It is easier to do this with the lake down but can be done from a boat if needed (driving the post not moving in a large oak). I would not use clorox . Most co-ops have or can get rotenone as can your county agent. ewest