Martin, to determinate your fish is difficult for the best experts. I asked Martin Hallmann to look at the pictures (amended by Tom) and he says (translated):
Its definitely not opallios, probably not spec. Sentang (which is more homogeneously coloured), so it maybe a local type of bintan (not Bangka, which is somewhat differently looking) or even a type of alfredi (which sometimes looses the red parts in its fins in becoming older; we have seen this many times). But since the fish is rather young this is unlikely. Probably a certain bintan-type.
But that means a lot of open questions. The fish may come from the southern parts of western Malaysia (unlikely), from the locations of bintan except Bangka, or especially from Sumatra where we have been known many similar bintan-like forms in the last years. In my view (PF) this is the most likely origin of the fish since from there many of the (undescribed) trade-Paros of the last years have come.
That’s clearly formulated, the problem has been confined, but is not solved up to now! So be proud: We do not know exactly what it is, but you probably keep one of the undescribed variants of bintan.