BLACK MOORS are long and torpedo shaped (more so than  those in this picture) with very large, flowing finage,  especially the ventral, caudal and anal fins.  They are matt  black with a bronze showing thru on the belly.  They DO NOT  decolorize.  They can get up to a foot in length.

DEMEKINS are high backed, short fish with shorter fins. The  underbody is silverish.  They may or may not be matt.