Why is my fish tank cloudy and is it over populated?

hi i have a 16 (uk gallon) fish tank and it has been set-up about a month and in the past week it has gone a murky gray colour only visible from the side of the tank? and is it over populated ?? i have 6 platys 5 neon tetras 3 black tetras? 1 rummy face tetra, 2 gold barbs and 2 plecos. Thanks in advance