Every decision you make about your pool starts with a test result. Which means a bad test result produces a bad decision, and then you're adding chemicals to fix a problem that doesn't exist.
I've watched people shock a perfectly fine pool because a two-year-old strip read low. That's $40 and an afternoon, spent on nothing.
Key takeaways
- Sample at elbow depth, away from returns, the skimmer, and wherever you last added something.
- Strips are acceptable for a quick pH check and unreliable for free chlorine and CYA.
- A FAS-DPD drop kit reads free chlorine to 0.2 ppm and pays for itself the first time it prevents a bloom.
- Reagents expire. A kit that's been in a hot shed for two summers is giving you fiction.
- Write readings down with the date. The trend is worth more than any single test.
Take the sample properly
More reading errors come from sampling than from the test itself.
Elbow depth. Roughly 18 inches down. Push the bottle in upside down, turn it over at depth, bring it up. Surface water is different — it's had UV on it, it's where oils collect, and it's not representative.
Away from the returns. Water coming out of a return has just been past your salt cell or chlorinator. It'll read high.
Away from the skimmer. That's water on its way out, carrying whatever's on the surface.
Away from where you last dosed. If you poured acid in the deep end yesterday, sample the shallow end.
With the pump running, or at least having run recently. Static water stratifies, and the reading from a pool that's been off for two days tells you about that one spot only.
Test promptly. Chlorine readings drift within minutes of pulling a sample, especially in sunlight. Don't leave it on the deck while you do something else.
What to buy
| Cost | Good for | Not good for | |
|---|---|---|---|
| Test strips | $15-$30 | Quick pH glance, rough alkalinity | Free chlorine, combined chlorine, CYA — genuinely unreliable |
| OTO/phenol red kit | $15-$25 | Total chlorine and pH, basic | Can't separate free from combined chlorine at all |
| DPD color kit | $30-$60 | Free and total chlorine, and bromine | Coarse color steps; Taylor's K-1005 scale stops at 5 ppm, the K-2005's at 10 |
| FAS-DPD drop kit | $70-$110 | Everything. Reads in 0.2 ppm steps, up to 20 ppm | Costs more, takes a couple of minutes longer |
| Digital photometer | $150-$400 | Removes color judgment, repeatable | Needs calibration, reagents still expire |
Why FAS-DPD is different. Instead of matching a pink shade to a color block, you count drops until pink turns clear. Counting is objective. Color matching in changing daylight, with a color-vision variation you might not know you have, is not.
It also reads to 20 ppm during a shock. Taylor's color-match DPD test stops at 10, and above that Taylor says the sample can bleach out and read falsely low.
The test kit pages here take four of the kinds in that table apart: AquaChek strips, Taylor's K-2005 DPD color kit, the K-2006C FAS-DPD kit and LaMotte's ColorQ photometer.
What you need
- Check price
AquaChek Select Connect test strips
Fine for a quick mid-week pH check. Not for decisions that cost money.
- Check price
Taylor R-0871 FAS-DPD reagent
The refill that runs out first. Buy the bottle, not a whole new kit.
What to test, and when
| Test | Frequency | Why |
|---|---|---|
| Free chlorine | 2× weekly in season | The one that actually protects you |
| pH | 2× weekly | Drives chlorine effectiveness and comfort |
| Total alkalinity | Weekly | The buffer holding pH still |
| Combined chlorine | Weekly | Above 0.5 ppm means shock |
| Calcium hardness | Monthly | Surface and equipment protection |
| Cyanuric acid | Monthly | Sets your real chlorine target |
| Salt (salt pools) | Monthly | Cell performance and longevity |
| Everything | After rain, after a party, after topping up | All of it moves |
That's roughly five minutes twice a week and fifteen once a month.
Reagents expire, and it matters
This catches everybody eventually.
Reagents go off, and heat is the enemy: Taylor says to store them between 36°F and 85°F. A kit that's spent two summers in a shed is giving you numbers, and those numbers are fiction.
Replace reagents annually. Buy refills rather than a whole new kit — much cheaper.
Store the kit indoors. Not the shed, not the pool box. Inside, where it's cool.
Signs your reagents have gone: readings that don't respond the way they should, chlorine that reads zero when you know you dosed yesterday, colors that develop oddly or fade fast.
Sanity check twice a season. Take a sample to a pool store and compare their numbers to yours. If they disagree substantially, one of you has a problem, and it's worth finding out which.
Taking a sample to the store, and reading what comes back
Free store testing is a genuinely useful cross-check, and it is worth knowing how to get a good result out of it.
Collect it properly — same rules as above, elbow depth, away from returns — into a clean bottle rinsed with pool water rather than tap water. Dish soap residue in a reused bottle affects several of the tests.
Take it in cool, and take it in soon. A sample that has sat in a hot car for two hours has lost chlorine and warmed enough to skew the CYA turbidity test. If you cannot go straight there, put it in a cooler.
Read the printout with a little skepticism. Store systems are usually photometers, accurate when calibrated and maintained, and stores vary in how diligently that happens. More to the point, the printout generally ends with a list of products to buy, generated by software that knows nothing about your CYA history or the fact that you dosed yesterday.
Use the numbers, ignore the shopping list, compare against your own kit. Two readings that agree give you confidence in both. Two that disagree tell you to find out why before acting on either.
The tests you do not need monthly
Three get sold as routine and are not.
Phosphates. Worth testing when chlorine demand is inexplicably high, or when algae keeps returning in a pool whose chlorine is genuinely correct. Not worth a monthly slot, because phosphate at ordinary levels is not a problem in a properly chlorinated pool — it is food that never gets eaten.
Metals. Test on a new fill, after topping up from a well, and if you see staining. Otherwise metals do not appear from nowhere.
Total dissolved solids. Largely a curiosity on a residential pool. It rises slowly, and the number alone does not tell you what rose — which is why the individual tests are more useful than the aggregate.
The specific mistakes
Testing CYA in cold water. The turbidity test is temperature-sensitive. Let the sample come to roughly room temperature or you'll read low and under-chlorinate all season.
Reading the CYA tube wrong. Waist height, indirect daylight, glance down at it. Not six inches from your face in direct sun.
Not diluting a high CYA sample. Most tubes top out around 100. Above that, dilute 50/50 with distilled water, test, and double the result.
Testing right after adding something. Give it a full turnover. Testing an hour after dosing tells you about the water near the return, not the pool.
Testing combined chlorine after MPS. Non-chlorine shock reads as combined chlorine for a day or two. Wait 48 hours.
Trusting the salt panel. A salt system's salt readout is a calculated estimate and it drifts as the cell ages. Verify before adding salt.
Write it down
Single readings tell you where you are. A log tells you what your pool does.
Date, free chlorine, pH, alkalinity, plus monthly calcium and CYA. A notes app is fine, or the test log, which keeps each test on your phone, charts it and turns the latest into a plan. After a month you'll know whether your pH climbs on its own, how fast you lose chlorine in a heatwave, whether your fill water is hardening the pool over time.
That's the difference between reacting to your pool and running it.
Go deeperWhat to do with those numbersTarget ranges, the order to adjust in, and the doses that work.Testing kit
- Check price
Taylor K-2006C test kit
Drop-count titration rather than color matching, and it tests CYA properly.
- Check price
Taylor R-0871 FAS-DPD reagent
The refill that runs out first. Buy the bottle, not a whole new kit.
- Check price
AquaChek Select Connect test strips
Fine for a quick mid-week pH check. Not for decisions that cost money.
Frequently asked questions
How often should I test my pool water?
Free chlorine and pH twice a week during the season and weekly off-season. Total alkalinity weekly. Calcium hardness and cyanuric acid monthly, plus after adding significant fresh water. Test everything after heavy rain and after any heavily used pool day.
Are pool test strips accurate?
For a rough pH check, they're acceptable. For free chlorine above about 5 ppm, for combined chlorine, and especially for cyanuric acid, they're unreliable — the color blocks are too coarse and the pads degrade in the bottle. Decisions worth money should be made on a drop-based kit.
What is the best pool test kit?
A drop-based FAS-DPD kit. It reads free chlorine by drop-count titration rather than color matching, which means readings in 0.2 ppm steps up to 20 ppm, twice the range Taylor gives its color-match DPD test. Expect around $90 to $100, and budget for fresh reagents each season.
Where should I take a pool water sample from?
Elbow depth, roughly 18 inches down, away from returns, away from the skimmer, and away from wherever you last dosed anything. Surface water and water directly in front of a return aren't representative of the pool as a whole.
How long do pool test reagents last?
Go by the best-by date on each bottle. Taylor's instruction card says to replace reagents once a year, and its FAQ says to store them between 36°F and 85°F, out of direct sunlight and away from pool chemicals. If your readings have gone strange and nothing about the pool has changed, replace reagents before you chase a phantom problem.
