Definitely water sprite. Grows like a weed, and often classified as one, make sure it stays in your tank and waste water doesn't get to a watercourse if you're in part of the US with the weather for it (I believe California may be the issue), it'll grow from almost nothing sometimes. It does indeed grow better rooted, and will show emersed growth if it reaches the surface. Shrimp seem to love it, but it can get scraggy if it gets too dense.