Are you sure?Najas indica is not guppy grass....Guppy grass is najas guadalupensis....
This pic looks like najas indica. Prettier, not as invasive...If the crown tops are thicker and more rigid, it is Najas guadalupensis or "guppy grass"
Check PlantGeek.net - Your Aquatic Plant Resource and look up Najas. You can differentiate from the two based on real life appearance. The pic isn't detailed enough so you have to take it from here.