[Peter Finke]

Wojtek, you are perfectly understood. Be welcome!!
Your fish might really be filamentosus, your description mentions the clear filament at the end of the male’s caudal fin. And there must be a colouration in that fin, reddish at the centre and bluish at the margin. The whole caudal is not rounded, but slighty elongated.
It is very well possible that the trade has mixed up two different species. That would be rather dangerous because we cannot idebtify the females very well. But at first we have to wait and let the animals develop.