Metro should be dosed daily for 12 days. This actually looks more like either intestinal lining or worm poo to me. If you don't have a microscope to tell you exactly what you are dealing with I'd worm with Absolute wormer (Prazi just gets tape worms). It's just a 1 day treatment. If that doesn't do the trick treat with metro for 12 days with a 60% WC every day during treatment.
It's best to add Prime straight to the tank either just before filling or while filling the tank. Other than the fact that 30% WC daily would be better I can see nothing else you are doing wrong. Your fish looks stunted. It may have come to you with parasites.