I think a lot of your problem has already been addressed a couple of times in earlier posts in this thread. When you have high cya like you do, you aren't going to kill any algae if you let your chlorine levels drop too low. You need to test 3x a day and each time bring your chlorine back up to shock level. Letting it continue to drop too low is going to keep you from getting this problem solved.

http://www.poolforum.com/pf2/showthread.php?t=365