Also, to add to what Janet told you, if the directions on the label tell you to broadcast it across the pool, don't do it. You don't want acid laying on your pool floor. Either add it to your skimmer and keep the pump running 24/7 for 4 or 5 days afterwards with no backwashing, or else put it into an old sock and hang it in front of a return jet. Give it a squeeze every now and then to help it dissolve faster.

CYA is kind of like sunscreen for your chlorine. If you don't have any in the water, you won't be able to keep chlorine in the pool. We typically recommend a CYA level of 40-50 for most pools.