mine is a 20g fresh water tank wiht 4 zebra danios, 3 large tigerbarbs, 4 plaites (2 died in the past 3 weeks) they all get along well, don;t even care abt others....my platies got a black spot on top of their mouth, and its getting bigger every day, whn i bought them they didn;t have these spots....the plaites died had these spots whn they were dead, is tht black spot some kinda of disease ? my tank is 3 months old and i got diatioms...any idea ? if its disease hw do i treat ?