We have Helanthium tenellum tenellum from South America (Narrow leaf) and the Helanthium tenellum parvullum from North America. (Broadleaf)
The narrow-leaf tends to grow a tad taller with red colour in submersed condition. It's also a tad more difficult to grow. The Parvullum is easier and stays quite smaller submersed.
In emerged form, they are quite difficult to keep apart. Then a flower will be conclusive, the parvullum has a much smaller flower.
I'm currently growing the parvullum.
The one with the red arrow is a fully transitioned over 6 months old submerged form. NExt to one planted 2 weeks ago still in transition.
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From this pot.
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You can clearly see that mine has a shorter and tad broader spear formed leaf than yours. My best guess is you likely have the South American HT. But as said it can be deceiving, to be absolutely sure you would need a flower or wait till it fully transitioned. In high light and ample CO² the SA HT will have red colours in it and grow over 5cm tall. The NA HTP rarely grows taller than 5cm submersed.