They are indeed slimmer than the other paros (some even like parvulus, but most are higher built), they have somewhat different stripes, and their colouration is really different. They have dark reddish fins with a distinct black spot in the dorsal fin.
What species other than sumatranus has a conspicious black dorsal spot? I only know it from sumatranus and allani, of which the latter seems very improbable.
If they are that rare nowadays I’ll get me some. I couldn’t clearly distinguish female sumatranus, do they have colour in their fins or are they clear like bintan?