Your Test Says the Chlorine Is Fine. So Why Did the Pool Turn Green?

Almost every pool owner has had this week. You test on Tuesday, the chlorine reads right where it should be, you feel good about it. By Saturday the water has a green cast, the walls feel slick, and you're standing there wondering what you did wrong.

Usually the answer is nothing. You did test. The problem is that the test didn't tell you what was actually happening — and in summer, that gap opens up fast.

Six Problems Pool Owners Blame on Chemistry That Are Really a Testing Problem

These come up constantly. See if any sound familiar.

Every one of those is a person doing the right thing and getting bad information back. That's worth separating out, because "I need to try harder" and "I need to be able to see what's happening" are two very different fixes.

Why Summer Makes All of This Worse

Chlorine loss in hot weather isn't one problem. It's several stacked together, and they peak at the same time.

• UV burn-off. Sunlight breaks down free chlorine. Cyanuric acid slows it down but never stops it — and as CYA climbs, less of your measured chlorine stays in the active form that actually sanitizes, so the same reading does less work.
• Warmer water. Every reaction speeds up, including the ones you don't want. Bacteria and algae reproduce faster at 88°F than at 72°F, so the same 3 ppm is doing more work per hour just to hold even.
• Bather load. Sunscreen, sweat, and body oils consume chlorine and turn it into the combined chlorine that makes eyes burn.
• Evaporation. Water leaves; calcium, salt, and cyanuric acid stay. Then you top off and add more on top. Hardness climbs all summer whether you add anything or not. If you're topping off constantly, our guide to automatic pool fillers and float valves covers keeping the level steady.

Storm season piles on. Whether it's a Southwest monsoon, a humid Gulf Coast stretch, or an afternoon thunderstorm line, a storm dumps organics and dust into the water, dilutes your chemistry, and spikes demand all at once. Our guide to keeping a pool clear during storm season covers the recovery.

How Much Can Hide Between Two Readings

Back to that first row in the table, because it's the one that catches the most people and the one nobody explains.

Every test method has a smallest step it can display. A strip has to jump from one printed color to the next. There's no in-between. So the real question isn't "is my test accurate," it's "how much can my pool change without my test changing?"

Run the numbers for your own pool. Say it burns about 2.5 to 3 ppm of free chlorine across a peak-summer day, with most of that during roughly ten hours of strong sun. That's somewhere near 0.25 ppm an hour at the worst part of the day. Now compare that to what each method can show:

That loss rate is an example, not a rule. Yours depends on CYA, sun exposure, bather load, and whether you run tabs or salt. Plug in your own numbers — that's the point.

Look at the top row again. A strip's smallest possible step can swallow half a day of chlorine loss. You can test at noon, read "3 ppm," test at five, read "3 ppm," and have genuinely lost a full point in between. The strip isn't broken. It just can't see that small.

This Is Worst Exactly Where It Matters Most

Combined chlorine — the burning-eyes problem from row two — is the hardest thing to measure and the most important thing to catch early. You don't measure it directly. You measure total chlorine, measure free chlorine, and subtract.

So if each of those readings is uncertain by about half a part per million, the subtraction stacks that uncertainty. Your combined chlorine answer can be off by close to a full point, which is twice the size of the 0.5 ppm threshold you're trying to detect. A reading of 0.4 and a reading of 1.4 can look identical. One means you're fine. The other means you're shocking tonight.

What Actually Makes a Kit Worth Owning

Working backward from that table, a kit that solves these problems needs five things:

• Chlorine measured by counting, not color-matching. A drop-count titration ends with a sharp flip from pink to clear. It either happened or it didn't — nothing to squint at, and two people get the same number.
• Free and combined chlorine, separately. Otherwise the burning-eyes problem stays invisible until it's obvious.
• A cyanuric acid test. Plenty of budget kits skip it. It's the most common reason a pool with chlorine in range still grows algae, and you can't see it coming without the test.
• Acid and base demand. Not just "your pH is high," but roughly how much it'll take to fix — which is what stops the overshoot-and-correct cycle.
• Individually replaceable reagents with dates on them. You burn chlorine reagent fastest. Replace that bottle, not the whole kit.

The Taylor complete FAS-DPD kit hits all five, which is why it ends up on service trucks and why DIY owners who've been burned by a green pool tend to land on it. Full contents and current pricing are on the product page.

Where It's Honestly Not the Right Call

It takes a couple of minutes instead of five seconds. It costs more up front than a bottle of strips. The reagents expire, so it's a real maintenance item. And there's about an afternoon of learning curve.

If you've got a lightly used pool in a mild climate and your water has never given you trouble, strips may be genuinely enough. The case for stepping up is strongest when the pool is fighting real summer heat, a heavy bather load, or a storm season — which is exactly when the problems in that first table show up.

How Often to Test Once It Gets Hot

Not sure what your target numbers should be? Our beginner's guide to pool chemicals lays out the ranges by region and climate.

One habit worth building: test at the same time of day. Free chlorine at 7 a.m. and free chlorine at 4 p.m. are two different numbers on the same pool. Pick one and stick with it, or you'll spend the summer chasing swings that aren't real.

Getting Readings You Can Trust

• Don't store reagents in the garage or the truck. Heat degrades them, and a degraded reagent reads wrong without telling you. Keep the case indoors and check dates each spring.
• Pull your sample elbow-deep, away from returns. Surface water beside a return jet isn't your pool.
• Hold dropper bottles straight up. Tilted bottles make smaller drops, and on a titration every drop counts.
• Rinse test cells with pool water, not tap.
• Check the filter before you blame the chemistry. If combined chlorine keeps climbing while free chlorine keeps dropping, a loaded filter is often part of it — poor circulation makes dead zones where chlorine never reaches. Fresh replacement filter cartridges fix more "chemistry" problems than people expect, and our walkthrough on cleaning versus replacing cartridges covers the difference.

One Extra Wrinkle for Salt Pools

A salt chlorine generator produces on a schedule, not on demand. When summer demand spikes, the cell can run at 100% and still fall behind — and you won't know unless you're testing. The first visible sign is usually a green tint on the steps, which is a week too late. If output has quietly dropped off with age, our salt cell comparison by brand covers replacement options.

Common Questions

Why did my pool turn green when the chlorine tested fine?

Free chlorine was most likely sliding down all week in steps too small for the test to display. Every method has a smallest change it can show, and a strip often can't register a change under a full part per million. In peak summer a pool can lose around a quarter ppm an hour during strong sun, so the reading stays identical for days while the level genuinely drops.

Why do my eyes burn if the chlorine reads normal?

That usually means combined chlorine is high — the chlorine has been used up, not overdosed. Cutting back at that point makes it worse. Combined chlorine is total minus free, so both readings have to be precise enough for the difference to mean something.

Why does my pool keep growing algae when chlorine tests in range?

Cyanuric acid has usually climbed high enough that less of your measured chlorine is in the active sanitizing form. The test still reads free chlorine — there's just less of it working at any moment. Trichlor tablets add stabilizer every time they dissolve, so CYA creeps up all season. Once you know the CYA number you can judge whether your chlorine target should be higher than the standard 2–4 ppm.

How often should I test in summer?

Free chlorine two to three times a week, or daily with heavy use. Combined chlorine and pH weekly to twice weekly. Alkalinity every two weeks, hardness monthly, CYA monthly if you run tablets. After a storm, test that evening and again the next morning.

Are test strips accurate enough for a home pool?

For a lightly used pool in a mild climate that's never given you trouble, often yes. The limitation is how small a change they can display, since a strip jumps from one printed color to the next with nothing in between. A drop-count titration reading in 0.2 ppm steps catches changes a strip can't, which matters most under heavy heat, high bather loads, and storm season.

The Short Version

If your water keeps surprising you, the problem usually isn't effort. It's that you're making decisions on numbers that can't move fast enough to warn you.

Testing isn't about getting a perfect reading. It's about catching a change while it still costs a jug of liquid chlorine instead of a weekend. We ship nationwide with free shipping in the US, and orders leave within one business day.