That's most definitely not hornwort. The "needles" on hornwort are stiff and will kind of poke you when you touch it. Those leave look like they have "give" to them, so I'd say either parrot feather, as someone suggested or some type of myriophyllum. It will be really hard to tell specifically without a close up view of the actual leaves to see how they are formed and divided.
Daayda, look on this site under "Tall Aquatic Plants" and see if any of those look like your plant close-up if you cannot get a clearer picture: http://www.aquahobby.com/e_garden.php
Look for Common Name: Milfoil
If you got this from petsmart it is frill(Myriophyllum Simulans). Thats where i got mine from and likes exactly like that and is what i came up with looking at the net for the plants i had.
Looks like Limnophila aquatica which is also known as Ambulia (not the Asian Ambulia though). I'm almost certain thats what it is. L. aquatica has a softer look than Myriophyllum (parrots feather).