"Subulata" I assume is Sagittaria subulata. And is "japonica" perhaps Blyxa japonica? The genus and species epithet are needed for identification.
It could be either, but of these two (if I'm correct in my assumptions) Blyxa japonica might be more likely, but the photos remind me of a thin-leaf Helanthium tenellum which can be very, very similar to the Sagittara subulata. These two vary depending upon growing conditions.