It has a reddish stem so it is not coontail. My dad's old pond had both Eurasian Watermilfoil and coontail, although they look kind of similar they are easy to tell apart looking at them side by side, stems are different color and the leaf pattern is very different. Coontail won't be rooted to the bottom and will be thick from the surface to the bottom of the pond. Milfoil will be rooted to the bottom and "leafy" on the top half only, the bottom half is all stem. I'm thinking Eurasian Watermilfoil.