CYA / Pool Stabilizer Series

High CYA in a pool: symptoms, causes, and how to fix it

High CYA is one of the most insidious pool problems because it mimics other issues. The pool has chlorine — the test strip says so. You're adding the same amount you always have. But the pool keeps getting green, or the water looks dull, or chlorine burns through faster than it should. The number you're not looking at is CYA.

Understanding what high CYA actually does — and what to look for — is the first step to diagnosing a problem that most pool owners spend entire summers fighting with the wrong treatments.

What counts as high CYA?

For a regular chlorine pool, CYA above 80 ppm is generally considered too high. Above 90–100 ppm, the practical problems stack up fast. Some pool professionals set the action threshold at 70 ppm to maintain a comfortable safety margin, since CYA tests are only accurate to ±10 ppm — a reading of 70 ppm could be anywhere from 60 to 80 ppm.

Signs that CYA may be too high

High CYA rarely announces itself with an obvious symptom. Instead, it shows up as a pattern of problems that seem unrelated:

• Algae despite "adequate" chlorine. You test FC and it reads 2–4 ppm — sounds fine — but the pool goes green anyway. At CYA 90, the minimum FC is 6.75 ppm. At 3 ppm FC, you're well below the sanitization threshold, regardless of what the strip shows.
• Chlorine burns off unusually fast. You add a normal dose in the evening and it's near zero by morning. High CYA doesn't cause this directly, but if you're dosing based on a target that's too low for your CYA level, you'll exhaust your FC before the next dose.
• Shock doesn't work. You shock the pool, it clears briefly, then algae comes back in a day or two. This often means you didn't reach the CYA-based shock threshold (CYA × 40%). At CYA 100, shock requires FC of 40 ppm — most standard shock doses don't come close to that.
• Green pool despite clear water. Early algae growth can make water appear slightly hazy or tinted before it turns visibly green. If chlorine doesn't hold and the water looks off, check CYA before chasing other causes.
• Pool store says "chlorine demand." This is often the diagnosis when everything looks normal but the pool won't hold chlorine. It's not wrong — chlorine demand does exist — but in many cases the underlying driver is CYA that has made the apparent FC target inadequate.

How CYA gets too high

CYA doesn't break down, evaporate, or get filtered out. It only leaves the pool through water removal. Every source that adds CYA contributes permanently — it accumulates through the season with no natural off-ramp.

The dominant cause is trichlor tablets (the standard 3-inch pucks used in floaters and automatic chlorinators). Each tablet is approximately 90% trichlor by weight and adds both chlorine and a significant amount of cyanuric acid as it dissolves. Using tablets as the primary chlorine source through a full summer can push CYA from 40 to 80–90 ppm or higher — quietly, without any dramatic change you'd notice.

Other contributors: top-off water from wells or municipal supplies doesn't typically contain CYA, but partial refills after backwashing or splash-out reduce pool volume and concentrate everything — meaning each subsequent tablet dose adds CYA to a smaller volume of water, slightly accelerating the rise. For more on which products add CYA, see does liquid chlorine raise CYA.

Why high CYA is a chlorine problem, not a CYA problem

It's tempting to think of high CYA as a stabilizer problem to solve by lowering stabilizer. But the more useful frame is: high CYA means your chlorine target is wrong.

CYA binds to free chlorine, holding it in a UV-protected but less active form. The higher the CYA, the greater the fraction of FC that's bound and less effective. To compensate, you need more total FC. The minimum FC is approximately CYA × 7.5% for regular pools:

Most pool owners maintain FC in the 2–4 ppm range by habit. At CYA 50, 3 ppm FC is just below minimum. At CYA 90, 3 ppm is less than half the required minimum. The pool appears chemically maintained but is chronically under-sanitized. Algae fills the gap. See the full FC/CYA chart for every level.

The shock problem with high CYA

When algae appears, the instinct is to shock. But shocking a pool with high CYA is where the problem becomes most visible.

Effective shock requires raising FC to CYA × 40% — the threshold where chlorine destroys algae and combined chloramines faster than they form. At CYA 80, that's 32 ppm. At CYA 100, it's 40 ppm.

A standard bag of cal-hypo shock raises FC by roughly 5–7 ppm in a 10,000-gallon pool. To reach 40 ppm FC in a 15,000-gallon pool, you need to add approximately 6–8 lbs of shock — and then hold it there, re-dosing every few hours as FC drops. Most pool owners shock once with one bag, get nowhere near the shock threshold, burn through a lot of chlorine, and end up back where they started within 24–48 hours.

How to fix high CYA

The only reliable fix is dilution: drain a portion of the pool and refill with fresh water.

The drain fraction needed equals (current CYA − target CYA) ÷ current CYA. To drop from CYA 80 to 40 ppm, drain 50%. To drop from 100 to 50, drain 50%. Use the CYA dilution calculator to find the exact gallons for your pool size and target.

After refilling, adjust your FC target to match the new CYA level before swimming, and switch to liquid chlorine or cal-hypo going forward to prevent CYA from climbing again. For the complete step-by-step drain process and what to rebalance afterward, see how to lower CYA in a pool.

What about CYA reducer products?

Several products are marketed as chemical CYA removers. Independent testing has not shown consistent, reliable effectiveness. Pool professionals use drain and refill because the result is predictable: drain X%, get X% less CYA. There's no waiting to see if a product worked.

Preventing CYA from climbing again

After draining to reset CYA, the choice of chlorine source determines whether CYA climbs again. Trichlor tablets will push CYA back toward 70–80+ ppm over another full season. Liquid chlorine and cal-hypo add none.

The practical approach many pool owners use: trichlor tablets at season startup to raise CYA from near-zero to 30–40 ppm, then liquid chlorine for all daily chlorination through the rest of the season. Test CYA monthly. If it approaches 60–70 ppm, a small partial drain (20–25%) keeps it in range without requiring the larger drain that 90+ ppm demands.

Frequently asked questions

What happens if CYA is too high in a pool?

High CYA raises the free chlorine level you must maintain for effective sanitization. At CYA 80, the FC minimum is 6 ppm. At CYA 100, it's 7.5 ppm — and shocking requires holding FC at 40 ppm. Most pool owners don't raise their chlorine target as CYA climbs, so the pool runs chronically under-sanitized even when it tests positive for chlorine. Algae follows, often repeatedly through the summer.

What CYA level is too high?

For regular chlorine pools, CYA above 80 ppm is high. Above 90–100 ppm the problems are severe — shock levels become extreme and test kits lose accuracy. SWG pools typically run 60–80 ppm intentionally, which is appropriate for that pool type.

How does CYA get too high?

Primarily from trichlor tablets used as the main chlorine source. Every tablet adds both chlorine and CYA. Over a full season, CYA can climb from 40 to 80+ ppm from tablets alone. CYA does not break down or evaporate — it only leaves through water removal. Dichlor granules also add CYA, at a lower rate.

How do I fix high CYA?

Drain a portion of the pool and refill with fresh water. No chemical reliably removes CYA. Drain fraction = (current CYA − target CYA) ÷ current CYA. After refilling, switch to liquid chlorine or cal-hypo. See how to lower CYA for the complete process.

Can high CYA cause algae?

Yes — this is the most important practical consequence. When CYA rises and FC is not adjusted upward to match, the pool runs below its required minimum FC. A pool with CYA 90 and FC of 3 ppm tests positive for chlorine but is under-sanitized. Algae can establish and grow in conditions that appear chemically adequate on the test strip. This is why repeated algae problems in pools that "have chlorine" are frequently a high-CYA problem.

Keep reading

• How to Lower CYA in a Pool — the drain-and-refill process, step by step
• What Is CYA (Pool Stabilizer)? — how CYA works and ideal levels by pool type
• FC/CYA Chart — required FC minimums, targets, and shock levels at every CYA value
• CYA Dilution Calculator — exact gallons to drain for your target CYA reduction
• Does Liquid Chlorine Raise CYA? — which chlorine types add CYA and which don't
• How to Shock a Pool — why reaching the CYA-based shock threshold is the step most owners skip
• Pool Turned Green Overnight? — high CYA with insufficient FC is the most common cause
• Why Algae Keeps Coming Back — recurring algae is often a CYA and FC target problem, not a sanitizer problem