CYA is a stabilizer that helps the chlorine from being consumed by the sun. The higher the CYA level gets, the higher the chlorine level is needed to do its job. Most people here shoot for a CYA level of 30-50 ppm. If you look at the top of this thread, you will find Ben's best guess chart that shows you what chlorine levels you need for a given CYA level. Tri chlor pucks contain CYA so using them will cause your CYA level to increase and also make your pH decrease so you need to keep an eye on those things. A lot of people here use bleach because it adds chlorine without anything else.

By testing for combined chlorine (CC) or "used up" chlorine, you can tell when you need to shock your pool. CC can cause eye irritation and give off a strong chlorine smell. Raising the Cl to shock level gets rid of the CC.

By shocking once a week you might have been using more chemicals (costing more $$$) than was necessary.

Peter