When it's exposed to sunlight, chlorine breaks down and no longer works as an effective sanitizer. This process happens quickly, and on a sunny day, the chlorine level in a shallow pool can go from an acceptable 5 parts per million to near zero in a matter of just a few hours.
Cyanuric acid, commonly referred to as CYA, acts as a stabilizer that resists the effects of sunlight on chlorine. CYA works by forming a chemical bond with the chlorine, which prevents the chlorine from breaking down under ultraviolet light. With the proper level of CYA in the water, which is typically recommended to be between 30 and 50 parts per million in outdoor freshwater pools, the chlorine level will be much more stable and the amount of chlorine you'll need to add to maintain adequate sanitizer levels will be greatly reduced.
Indoor pools, which aren't exposed to sunlight, don't need CYA, and saltwater pools should have a CYA level of between 70 and 80 parts per million.
While stabilizers stop chlorine from breaking down, they also reduce chlorine's effectiveness as a disinfectant. Consequently, when there's CYA in the water, the chlorine level in the water needs to be higher than it would otherwise be in order to be effective. The recommended chlorine level depends on the CYA level; when CYA is at 30 parts per million, chlorine should be between 2 and 4 parts per million, but when CYA is at 50 parts per million, chlorine should be between 4 and 6 parts per million.
Maintaining the higher chlorine level will require adding more sanitizer, but the additional product required will be much less than the amount required to replace losses caused by UV exposure if you don't use stabilizer, and you'll have to add chlorine much less often.
Because CYA severely reduces the effectiveness of chlorine in killing certain bacteria common in spas and hot tubs, using CYA in spas is not recommended.
CYA stabilizers are available as standalone products, and they're also included in some chlorine-based sanitizer products. Chemical products called dichlor and trichlor contain both chlorine and CYA, and if you are using these products as sanitizers in your pool, you're unlikely to need to add supplemental CYA.
Test CYA levels weekly to ensure that they're in the recommended range.
If you need to add stabilizer to raise your CYA level, first determine how much stabilizer you need to add. In general, about 13 ounces of granular stabilizer will raise the CYA level of 10,000 gallons of water by 10 parts per million.
Add the stabilizer to the skimmer basket while the pump is running, and keep the pump running for 24 hours after you add the stabilizer. Don't backwash or clean the filter for a week after adding the stabilizer.
Stabilizers dissolve in the pool very slowly, and you won't be able to get an accurate test reading to show how much you've raised the CYA level for several days. Wait a week before testing the level again, and only then should you add more stabilizer if necessary.