Why Your Chlorine Keeps Disappearing (And What CYA Actually Does)

This is one of the most common support questions in pool ownership: "I added chlorine yesterday and it's already reading zero. What's wrong?"

Usually, nothing is wrong with the dosing. The chlorine is doing exactly what unprotected chlorine does in direct sun — it's getting destroyed by UV light before it ever gets a chance to sanitize anything.

Chlorine has a sunburn problem

Sunlight, specifically UV-B radiation, breaks apart the chlorine molecule before it can react with anything else in the water. Left completely unprotected, a pool in full sun can lose 90% of its free chlorine in under two hours. That's not a dosing failure — it's basic photochemistry.

Cyanuric acid (CYA) — sold as "stabilizer" or "conditioner" — works like sunscreen. It doesn't stop chlorine from sanitizing, but it forms a loose, temporary bond with the chlorine molecule that blocks most of the UV damage, releasing the chlorine to do its job when it's actually needed. No CYA, and every dose you add is racing the sun. Too much CYA, and it starts working too well — chlorine gets so buffered that it can't sanitize effectively at all, a state pool folks call chlorine lock.

The ratio that actually determines your target

Here's the number most test-strip charts don't tell you: your minimum effective free chlorine isn't a flat number. It scales with your CYA level. This is the guidance the CDC publishes for public pools, and it holds for residential ones too:

CYA (ppm)	Minimum FC (ppm)	Target FC (ppm)
0 (none)	1	2
1–15	2	3
16–30	3	4
31–40	4	5
41–50	5	6
51–60	6	7
61–70	7	8

Read a flat "3 ppm is fine" off a generic chart with CYA at 60 ppm, and you're actually under-sanitizing — that reading needs to be closer to 6–7 ppm to do the same job. This is the single most common reason a pool that "tests fine" still turns green within a few days of hot, sunny weather.

Where CYA actually comes from

You don't dose CYA every week — it builds up gradually, mostly from two sources:

• Stabilizer added directly (granular cyanuric acid, dissolved slowly in a sock — never poured straight into the skimmer, it can bleach vinyl and settle undissolved on plaster).
• Stabilized chlorine products, mainly dichlor and trichlor. Every dose of these adds a little CYA along with the chlorine — trichlor especially, since it's roughly half stabilizer by weight. A pool that runs on trichlor tabs all summer, every summer, without ever diluting, is the classic way CYA creeps past 80–100 ppm over a couple of seasons.

If you're chasing chlorine that won't hold and your CYA is already at or above 50 ppm, more stabilizer isn't the answer — you're already past the point where it helps. The fix at that point is either diluting the water (partial drain and refill; there's no chemical that removes CYA) or switching your routine chlorine source to an unstabilized one like liquid chlorine or cal-hypo, so the level stops climbing further.

The practical takeaway

1. Test CYA when you set up your pool for the season, then again every month or two — it doesn't move fast.
2. Use your CYA reading to set your actual target FC, not a flat number off a strip chart.
3. If you're topping off chlorine constantly and CYA is under 30 ppm, add stabilizer before you add more chlorine — you're burning money into direct sunlight.
4. If CYA is already high and chlorine still won't hold, stop using stabilized chlorine products and consider a partial drain.