There are 49 species of barbula recorded here in India. Almost all are purely terrestrial. Some which are said to be aquatic are either hard to find or hard to id. So they all have to be tested and tried.
I very rarely check ukaps these days, maybe once every few months so it was quite a coincidence that I saw this thread revived from so long ago at the top of the page tonight. In the end I bought some of this moss, I think from the green machine, and it does grow very well submersed with co2.
You need to be quite strict with trimming it otherwise it can get a little stringy but it's well worth it purely because it looks so much like a terrestrial moss in its mounding growth habit.
I don't have any growing submersed right now as I'm about to get a new aquarium after my flatmate smashed my planted tank but it's also flourishing in my emersed growing terrarium alongside some mini fissidens and looks much the same emersed as it does submersed. Surprisingly it actually seems to grow faster submerged even though all the evidence points to it being a terrestrial moss.