It looks like a Hygrophila to me. It also looks like emersed growth, which makes it harder for me to identify. Hygrophila are not particularly easy to identify, and the scientific names haves changed often over the last few decades.
The closest picture I can find to it on the site tropica.com is for what they call Hygrophila corymbosa 'Angustifolia'.
Perhaps other people will be able to be more definite!
Mark