Not 100% sure but it could be Hygrophila polysperma
At first I was going to suggest java fern but then it looks like there are offshoots from the primary stalks which doesn't make sense for the java. Perhaps some sort of sword, of which type I wouldn't be sure. I'm certain someone will be along to advise more accurately.
I don't think it's hygrophila polysperma because it doesn't match up with the pics.
Maybe its Hygrophila corymbosa?
Hygrophila corymbosa is the quickest growing plant I've ever had