CYA = 20?

Ok, shock to 10 or 12 and hold it there until algae is dead, then maintain between 3 and 6ppm.

You should raise CYA to 30 or 40--Use 1/3 to 1/2 of what you calculate you'll need to get it there.

Or you can use Tri-chlor pucks/sticks --they'll add it.