CYA binds to free chlorine, holding it in reserve so UV doesn't destroy it. At 30–50 ppm, this is protective. Above 80–100 ppm, so much chlorine is bound that very little is available to actually sanitize — a condition called "chlorine lock."
The paradox: the pool tests at "2 ppm free chlorine" but algae is growing, bathers are getting irritated, and nothing seems to work. The test measures bound + unbound chlorine together — but only unbound chlorine sanitizes. At 100 ppm CYA, you may need 10+ ppm free chlorine to achieve the same sanitation as 1 ppm would provide in an unstabilized pool.
Fix: test CYA. If above 80 ppm, do a 50% partial drain and refill. No chemical removes CYA — dilution is the only solution.
pH determines what fraction of free chlorine is in the active hypochlorous acid (HOCl) form:
| pH Level | Active Chlorine (HOCl %) | Effect |
|---|---|---|
| 7.0 | ~75% | Very effective but corrosive |
| 7.2 | ~65% | Ideal — effective and safe |
| 7.4 | ~55% | Good — slight activity reduction |
| 7.6 | ~45% | Acceptable |
| 7.8 | ~33% | Marginal — poor shock effectiveness |
| 8.0 | ~21% | Very poor — most chlorine is inactive |
If pH is above 7.8 and you're wondering why chemicals don't seem to work — this is why. Lower pH to 7.2 with muriatic acid before adding shock or making significant chemical additions.
Pool chemistry is weight-based — the amount of chemical needed depends on pool volume and the severity of the problem. Underdosing is common because:
The solution: calculate your pool volume accurately, use the full recommended dose for the problem severity, and use un-stabilized calcium hypochlorite for shocking.
Chemicals added to a pool with inadequate circulation never reach the entire water volume. Dead zones in corners, under steps, and in shaded areas receive very little chemical exposure — algae and bacteria establish in exactly these locations.
Fix: ensure the pump runs at least 8 hours per day (more in summer), position return jets to maximize circulation, add a supplemental return jet or secondary skimmer if dead zones are chronic, and brush all surfaces when treating — physical disruption helps chemicals reach algae colonies.
Chemicals lose potency over time. Key degradation timelines:
The diagnostic sequence for chemicals that seem ineffective: (1) Test CYA — if above 80 ppm, drain first. (2) Test and adjust pH to 7.2. (3) Recalculate pool volume and use the correct dose. (4) Run the pump 24/7 during treatment. (5) Check chemical freshness. Work through this order before assuming the chemicals or the pool are broken.
SplashLens logs every chemical addition with before-and-after test readings — so you can see exactly which chemicals are producing results and which aren't, and identify patterns that point to a systemic issue like high CYA or chronic pH drift.
Open SplashLens Free →Fast chlorine depletion usually means low CYA (no UV protection), very high CYA causing excessive demand, heavy bather load, active algae bloom consuming chlorine, or high phosphates feeding algae. Test CYA first — it's the most common root cause in both extremes.
pH that won't hold is almost always an alkalinity problem. If alkalinity is below 80 ppm, pH is unstable. Fix alkalinity first (80–120 ppm with sodium bicarbonate), then adjust pH — it will hold much more stable once the buffer is correct.
Shock fails when: pH is above 7.8 (chlorine mostly inactive — lower pH first), CYA is above 80 ppm (drain 50% first), dose was too low (2–3 lbs cal-hypo per 10K gal for green algae), or shock was added during the day (UV destroyed it). Fix the conditions, then re-shock correctly.
Yes. Liquid chlorine loses ~50% strength within 30 days of opening. Cal-hypo granules lose 5–10%/year in proper storage — more with humidity or heat. Test strips degrade if left in an open container. Always use fresh chemicals and check expiration dates before treating a problem pool.
Yes — always add chemicals with the pump running for even distribution. Chemicals added to stagnant water concentrate locally and may be corrosive to nearby surfaces. Shock added near vinyl can bleach it. The pump ensures chemicals distribute throughout the entire water volume before they reach full concentration.