I'd also say H. chaophraya - I think they're specifically targeted for the trade. I had no idea that there was any trade in live specimens at all until coming here.
Based on this photo I suspect you're right in calling it Himantura polylepis (syn. H. chaophraya), but I wouldn't be able to exclude all other species with absolute certainty.
(e.g. I still haven't had the time to read up on the confusing H. uarnacoides complex, which includes several very similar species that may enter brackish/fresh water and sometimes are seen in the aquarium trade, often under the obsolete name H. bleekeri)
