You would be surprised, how many species, sub-species, varieties, cultivars.......and you name it.....that goes around within Hygrophila (and there are "new" Staurogynes, too). Same is true of Echinodorus, Rotala, Limnophila, Pogostemon, Chryptocoryne......(get the idea???)..........
You also must remember, that same plant (can) look very different, growing in different environments - your plant would most likely not look totally alike, if I grew it.........leaves are therefore not good/safe for ID - which is why botanists determine by flower.
Several quite similar looking plants have"passed through my hands", yes, but from the above you will now know, why I am quite reluctant to give a defenite ID from pic.s of leaves. There ate several kandidates; too many to choose from.
Like Farrell said, somewhere - it's often quite easy to tell, what a plant is NOT.....but also very often hard/impossible to tell exactly what it actually IS !!!!
- Sorry I can't do magic ......