Cyanuric Acid (Stabilizer)
The suns ultraviolet (UV) rays will make the chlorine in an outdoor pool dissipate quickly. In fact, an ideal level of chlorine in an unstabilized pool or spa can be lost in less than two hours on a bright sunny day, due to the UV rays of the sun. Cyanuric acid acts as a stabilizer that helps chlorine hold up better when exposed to the UV rays. You can think of cyanuric acid as blocking the effect that the sunlight has on breaking down the chlorinekind of like a sunscreen for your pool.
You should maintain an ideal level of cyanuric acid, 30 to 50 ppm (mg/L), to prevent rapid chlorine loss. If the cyanuric acid level is too low, you may need to add more to the water. However, be advised that cyanuric acid will make the pH of the water lower (more acidic), so you may have to adjust the pH upward as well.
On the other hand, too much cyanuric acid will reduce the beneficial effect of your chlorine, leading to stains or cloudy water. Some chlorine compounds already contain an amount of cyanuric acid. If you are using dichlor or trichlor as your primary chlorine sanitizer, you are already introducing cyanuric acid along with the chlorine. If the cyanuric acid level is your pool or spa is too high, you will need to partially drain and refill with fresh water.
When you first fill your pool or spa, test the cyanuric acid level until you have added enough to reach the ideal range. After that, test cyanuric acid a minimum of once a month throughout the pool season. If you are using dichlor or trichlor, you will have to test cyanuric acid more frequently to ensure that the level has not exceeded the upper limit.