Fish tank get dirty very quickly?

i i have a rectangular fish tank (11.5 inches tall and 17.5 inches wide front and back) _left and right 11.5 inches tall and 11 inches wide) and i have filled it with 8.2 inches of water how much gallons does it make and every week how much inches of water should i remove and how to avoid fish tank getting cloudy and dirty i have 4 fantail gold fishes and 2 angle fishes