Yes, agreed, Neil Frank's article is a very good read indeed.
However, back to the original question.
The E. tenellus
'micro' is exactly what is meant by Helanthium tenellum
Though I haven't grown the plant normally sold in the US under the name E. tenellus
I think it is safe to say that that in all likelihood is NOT Helanthium tenellum
. It should be called H. bolivianum
as all the other runner-making ex-swords are currently grouped under that name (save the H. zombiense
which is so rare that you won't have it) - even the E. angustifolius
. However, Samuli Lehtonen who published this new revision has publicly annouced his doubts about this.
He has said/ written that there probably are more than these 3 species of Helanthiums
out there. It is unfortunately near-impossible to tell a) which plants belong to which species (due to their high morphological plasticity, cf. Frank's article) and b) which name should go with what species. Both questions should be resolved before one can say a name for a Helanthium
sp. No one has studied these plants closely enough to say the last word on them.
That said, H. tenellum
as per Lehtonen (and the revisionists before him: Haynes & Holm-Nielsen, 1994) is easy to distinguish with its 1-2 mm wide submersed leaves that get that reddish hue in proper light. It doesn't get higher than 4'' normally, usually less. If you were to grow that plant emersed, you would get flowers that differ esp. in size and somewhat in shape of the other Helanthiums
(the others much more closely resemble each other).
Hope I managed to clarify the situation a bit there...