Yeah, it's an interesting conundrum, because the ornamental fishermen don't seem to actively target these less common knifefish species. Rather, at the times of year that they are abundant, I believe they incidentally catch some and keep them to sell. The problem (I believe) is that there is enough demand from a few Japanese exporters that they buy up almost all of the stock, so that there haven't been enough exports to really establish a market in the US.
This is just my take..would love to hear from someone that knows more about the industry.