Cyanuric acid is sunscreen for chlorine. Without it, unprotected chlorine in an outdoor pool loses more than half its strength in a few hours of direct sun. With it, chlorine survives the day.
The trap is that CYA also binds chlorine. The higher it goes, the more of your free chlorine is locked in a form that reads on a test strip and kills nothing.
The ratio that actually governs the pool
Free chlorine has to be roughly 7.5% of the CYA level to remain effective.
| CYA (ppm) | Minimum free chlorine | Target free chlorine | Shock level |
|---|---|---|---|
| 30 | 2 | 3–5 | 12 |
| 40 | 3 | 4–6 | 16 |
| 50 | 4 | 5–7 | 20 |
| 60 | 5 | 6–8 | 24 |
| 80 | 6 | 7–10 | 31 |
| 100 | 8 | 9–12 | 39 |
Read the last column. At 100 ppm CYA a proper shock is 39 ppm of free chlorine — a quantity most people will never add, which is exactly why high-CYA pools stay green through a whole summer of shocking.
How it gets high
Trichlor tablets. Each 1 ppm of chlorine from trichlor adds 0.6 ppm of CYA. A season of tablets on a 20,000 gallon pool adds 60–90 ppm, and nobody notices because the pool looks fine until it does not.
Dichlor does the same at 0.9 ppm CYA per 1 ppm chlorine.
"Stabilised shock" is dichlor with a different label.
It does not break down
CYA is chemically stable. It does not evaporate, it is not consumed by chlorine, and sunlight does not destroy it. It leaves by exactly one route: water leaving the pool.
Products sold as "CYA reducer" are either enzyme treatments with inconsistent results or flocculants that remove a fraction. Draining is the reliable method, and it is the cheap one compared to a season of ineffective chlorine.
How much to drain
Dilution is linear:
percent to replace = (1 − target ÷ current) × 100
From 100 ppm to 40 ppm: (1 − 0.4) × 100 = replace 60% of the water.
| Current | Target 40 ppm | Water to replace |
|---|---|---|
| 60 | 40 | 33% |
| 80 | 40 | 50% |
| 100 | 40 | 60% |
| 150 | 40 | 73% |
| 200 | 40 | 80% |
Do not drain a vinyl or fibreglass pool below the level a professional advises. A vinyl liner can shrink and fail to reseat; a fibreglass shell can float out of the ground in a high water table. Partial drain-and-refill in stages is the safe method, and in some regions it needs doing with the hydrostatic relief valve open.
The target range
30–50 ppm for an outdoor chlorine pool. 60–80 ppm for a salt pool, which needs more protection because the cell produces chlorine continuously at low concentration.
Zero for an indoor pool. There is no sunlight, so CYA does nothing except bind chlorine — and indoor pools have a different chlorine problem entirely.
Testing it properly
The turbidity test — the one where you fill a tube until a black dot disappears — is imprecise above about 100 ppm. If the dot vanishes immediately, dilute the sample 50/50 with distilled water, test, and double the result.
Test strips for CYA are notoriously unreliable. This is the one reading worth confirming at a pool store or with a proper kit before deciding to drain a third of the pool.
The diagnosis in one line
Chlorine reads fine, pool is green, and you have been using tablets. That is CYA until proven otherwise — and no amount of extra chlorine fixes it. Work out how many gallons you actually have first, because the drain percentage above is only as good as that number.