Clarifier should only be used when all your other water chemistry readings are perfect, you've added shock and still ahve cloudy water.
Clarifiers work by causing the suspended particualtes in the water to clump together so they fall to the bottom and can be vacuumed out. If you add too much clarifier it causes the particualtes to repell each other and stay suspended in the water - causing cloudy water.
Best advice - keep shocking and vacuuming/brushing until the water clers up.
I work for a pool company in Denver (Carousel Pools 303-422-7114)