Do the flowers have a white flower on the end of a single stem or do they have a single stem that branches into a number of flowers? If it is the latter it will be Sagittaria subulata which can grow tall in the right conditions, If it is the single flower it will be Valisneria which to my knowledge has no dwarf form. I have a dwarf sagittaria called Sagittaria microfolia which actually stays dwarf but when I have offered it on this site no one wanted it. (none available at the moment).