Cyanuric acid, often called stabilizer or conditioner, is an organic chemical compound added to pool water. It interacts with chlorine in two distinct and very important ways - it acts as both a buffer and protects chlorine from degradation by UV light.
Cyanuric acid, when dissolved in chlorinated pool water, forms various compounds with chlorine called chlorinated cyanurates. These are chemical compounds very similar to the original cyanuric acid where one of the hydrogen atoms has been replaced with chlorine. In this way, the cyanurate compound buffers the chlorine in your water. When the chlorine atom attaches itself to the cyanuric acid molecule, it is effectively “held in reserve” and will not participate in disinfection or chemical oxidation reactions. This makes the pool water much less harsh in terms of exposure of the swimmer to active chlorine compounds (hypochlorite and hypochlorous acid). It is important to stress this point - the cyanuric acid concentration determines the amount of active chlorine (hypochlorous acid + hypochlorite ion) available in pool water. This is why the CYA level is so critical to know and why TFP teaches that there is no such thing as an absolute FC level which is safe; the safe level of FC is determined by how much CYA is in the water. The entire pool industry gets this concept totally wrong when they state that a 1-3ppm FC is all you need. THAT. IS. WRONG! Your FC level is determined by your CYA level, FULL STOP! The higher your CYA level, the more FC you need in order to maintain a proper level of active chlorine.
Cyanuric acid and the related chlorinated cyanurates also protect chlorine from degradation, or loss, due to UV light. When chlorine in water is exposed to the UV light from the sun, the hypochlorite ion (OCl-) and hypochlorous acid (HOCl) both react with UV light and get degraded into oxygen gas and chloride ion (Cl-). Both types of active chlorine have different rates of extinction by UV light, but, on average, chlorine in water with no CYA present will have a half-life (ie, the time it takes for the initial concentration to decrease by half) of approximately 35mins. So if you put chlorine in your water and wait 2 hours, there will be less than 5% of the original dose left simply from loss to UV light.
When cyanuric acid is added to water and chlorine is able to bind to it, the half life of the chlorine increases to a maximum of 8.4 hours depending on how much CYA is present. The greater your CYA level, the less chlorine is lost. There are two reasons for this. First, the presence of cyanuric acid causes chlorine to become bound to it and held in reserve. Because of this, there is much less active chlorine (hypochlorous acid and hypochlorite) present in the water. Since there is less active chlorine, there is less chlorine to react with UV and, ultimately, lose. As well, chlorine bound to CYA is much more stable when exposed to UV light and will not readily degrade. Because of this, CYA is very good at protecting chlorine form UV loss. The CYA molecule, with no chlorine attached to it, also absorbs some UV and radiates it back at a different wavelength of light and so it, all by itself, will reduce the amount of UV hitting the water volume (as UV light passes into the water, it is lust as likely to interact with a molecule of CYA as it is with a molecule of chlorine). This inherent UV shielding effect is seen at higher levels of CYA.
You increase CYA by adding cyanuric acid, often sold as stabilizer or conditioner. CYA is available as a solid and as a liquid. The liquid costs a lot more (usually about 3X the price of granular solid CYA), and generally isn't worth the extra expense. Solid stabilizer is best added by placing it in a sock or nylon and hanging that sock in front of fast moving return. Alternatively, the sock can be placed into the skimmer basket BUT it should only be left in there if the pump is running. CYA is acidic when it dissolves into water and so it is not a good idea to leave the sock in the skimmer without the pump running. Pouring granular CYA into the pool directly is not recommended as it takes a long time to dissolve and the granules can sit against the pool surface creating locally low pH levels. The pump should be run for 24 hours after adding solid stabilizer and you should avoid backwashing/cleaning the filter for a few days if it is added to the skimmer. Solid stabilizer can take up to 24 hours to dissolve into the pool water no matter the method of addition and so it is important to wait for it to dissolve and for the pump to homogenize (mix) the pool water well before testing. It has been found that if you wait approximately 24-48 hours after the last bit of CYA has dissolved to test that those results will show up on a test. Be aware that the current test for CYA (melamine turbidity test) can only detect CYA difference of 10ppm at best. So if you are adding small amounts of CYA to your pool, the test might not show it.
In nearly all cases the best way to lower CYA is to replace water. If replacement water is extremely expensive or your local water utility has restrictions in place then you might want to look into a reverse osmosis water treatment.
