Pool Care & Chemistry
What the color of your pool is telling you
Water appearance is a symptom, not a diagnosis - but it narrows things down fast. Here is every common appearance and what it points at.
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The short version
- Color narrows it down; it does not diagnose. Every entry below ends in a test rather than a purchase.
- Green is sanitizer. Milky is filtration. Brown is usually organic or metal. Those three cover most of what you will see.
- Clear is not a diagnosis either. A pool with no chlorine looks exactly like a pool with plenty.
- Two checks answer most of it: free chlorine, and whether a rinse still restores the filter’s flow.
How to use this page
Find the appearance, read the likely cause, then do the check in the last column before buying anything. The single most expensive habit in pool care is treating a symptom that turns out to have a mechanical cause — a clarifier dosed into a blocked filter, or shock added to a pool whose pH means the chlorine cannot work.
| Appearance | Usually | Check first |
|---|---|---|
| Clear and blue | Normal — or no sanitizer at all | Free chlorine, always |
| Green tint, water clouding | Algae; sanitizer has failed | Free chlorine and cyanuric acid |
| Yellow-brown film on shaded walls | Mustard algae | Brush it; does it return in days? |
| Milky or hazy white | Fine particles the filter is not catching | Does a rinse restore the flow? |
| Cloudy right after shocking | Normal, temporary | Filter continuously; recheck tomorrow |
| Brown or tea-colored | Leaf tannin, or dissolved iron | Chlorine tablet held against a stain |
| Blue-green with staining | Copper, often from low pH | pH, and whether you use a mineral system |
| Clear water, colored stains only | Stain rather than water problem | The tablet test |
| White crusty deposits | Calcium scale | pH first, then hardness |
| Foam on the surface | Surfactants — product or swimmers | What went in most recently |
| Bubbles at the return jet | Air on the suction side | Water level and hose fittings |
Green: a sanitizer story
Green means algae is established and the chlorine lost. The recovery is always the same shape — brush every surface, shock heavily at dusk with the pump running, filter continuously until it clears, then dose a preventive algaecide last — and the order matters more than the products. Our algae guide is the sequence.
Before you start, check cyanuric acid. If stabilizer has climbed through a season of trichlor tablets, chlorine is bound and sluggish and will read respectably while achieving little — which is why some recoveries take a fortnight and others take two days. Our cyanuric acid guide covers the fix, which is dilution.
Yellow-brown rather than green, clinging to shaded walls and returning within days of being brushed off, is a different organism that tolerates ordinary chlorine levels — see mustard algae.
Milky white: a filtration story
Hazy water with correct chemistry is particles too fine for the filter to catch, or a cartridge loaded past the point where rinsing helps. The test is simple: if a rinse no longer restores the return flow the following day, the media is finished and no chemistry will fix it.
Change the cartridge, run the pump continuously, and reassess before reaching for a clarifier — a coagulant aggregates fine particles so the filter can catch them, which does nothing at all if the filter cannot. Our cloudy water guide is the diagnosis and the cartridge guide covers the interval.
One exception worth knowing: water frequently goes cloudy for a day or two immediately after a heavy shock, because you have just killed a great deal of material that the filter now has to remove. That is the process working, not failing.
Brown, blue-green and stains
These are the ones people most often treat wrongly, because organic and metal marks look similar and want opposite treatments. The test takes ten seconds: hold a chlorine tablet against the mark with a glove on for five to ten seconds. If it lightens, it is organic and chlorine will clear it. If nothing happens, it is metal or mineral — and shocking will oxidize the dissolved metal and set the color permanently into the vinyl.
Blue-green usually points at copper, which arrives either from a mineral system or from metal fittings corroding in water that has drifted below the CDC’s published pH range of 7.2 to 7.8. Rust-orange points at iron, often from the fill water. Our liner stains guide covers both, and chlorine alternatives covers why mineral systems carry that risk.
Foam and bubbles, which are different things
Foam is a persistent white head that drifts downwind and stays when the pump stops. It means surfactants — a quat algaecide, detergent on swimwear, or sunscreen and body products after a busy day. Our pool foam guide covers telling which.
Bubbles come from the return jet and stop within a minute of the pump stopping. That is air being drawn in on the suction side — a water level below the middle of the skimmer opening, a loose hose fitting, or a dry O-ring. Our hoses and fittings guide covers it and it has nothing to do with chemistry.
The two checks that answer most of this
If you take one thing from this page, take these. Free chlorine, because it separates every sanitizer problem from every mechanical one, and it is the reading that is invisible in the water. And whether a rinse still restores the filter’s flow, because that separates a chemistry problem from a filtration one.
Those two, fifteen seconds and one observation, land you on the right page nearly every time. Both are in the routine in our weekly schedule, and testing mistakes covers taking a sample that actually means something.
What to buy
Scores are our own reading of published specifications. They are not test results — we have not tested these products and we do not claim to.
The default first test kit
AquaChek Silver 7-Way Test Strips (100 count)
AquaChek
Seven readings in fifteen seconds, including the two that beginners never measure and then fight all summer: total alkalinity and cyanuric acid.
#ad how this is funded · price as of October 8, 2026
Key specs
- Tests
- Free chlorine, total chlorine, bromine, pH, total alkalinity, total hardness, cyanuric acid
- Free chlorine range
- 0-20 ppm
- pH range
- 6.2-8.4
- Total alkalinity range
- 0-240 ppm
- Cyanuric acid range
- 0-300 ppm
- Read time
- 15 seconds
- Count
- 100 strips
How it scoresour reading of published specs, not a test result
- Accuracy
- 7.0
- Ease
- 9.5
- Coverage
- 9.5
- Cost per use
- 9.0
- Value
- 9.2
Overall 8.9 / 10
What works
- Covers cyanuric acid, which most cheap strips omit and which is the reason chlorine stops working in a stabilized pool
- pH range spans 6.2-8.4, comfortably bracketing the CDC's recommended 7.0-7.8
- 15 seconds and a color chart printed on the bottle — no separate booklet to lose
- 100 strips is a full season of twice-weekly testing with room to spare
What doesn't
- Strips are a comparison against a printed color chart, so they are directional rather than precise
- Reading them in bright sun is harder than it should be
- The bottle must be kept sealed and dry or the strips degrade quietly
Skip it if: Fine adjustment. If you are chasing a stubborn problem and need a number rather than a color band, a liquid reagent kit is the tool.
What it costs to run: 100 strips at twice-weekly testing covers roughly two seasons for one pool. The real saving is upstream: knowing your alkalinity stops you buying pH adjuster you did not need.
One honest season: The cheapest setup that genuinely works for a season of real swimming. Nothing spare, nothing missing.
Specs from: CDC — home pool and hot tub water treatment and testing
Actually clearing a green pool
Calcium Hypochlorite Pool Shock
Various (compare available chlorine %)
Unstabilized, fast-acting and strong. This is the tool for an algae bloom precisely because it does not add more cyanuric acid to a pool that probably already has too much.
Key specs
- Type
- Calcium hypochlorite, unstabilized
- Adds stabilizer
- No
- Adds
- Calcium hardness
- Dosing
- By pool volume, per the bag
How it scoresour reading of published specs, not a test result
- Effectiveness
- 9.5
- Ease
- 6.5
- Water impact
- 7.0
- Cost per season
- 9.0
- Value
- 8.8
Overall 8.6 / 10
What works
- No cyanuric acid added, so it does not worsen the problem it is usually solving
- Fast and strong enough to actually break an algae bloom in one treatment when dosed properly
- Cheap per pound of available chlorine
What doesn't
- Adds calcium hardness, which matters in hard-water areas
- Must be dissolved properly or it will bleach a vinyl liner where it settles
- Needs careful storage — it is a genuine oxidizer, not a household product
Skip it if: Routine daily chlorination. This is the emergency tool; tablets are the routine.
What it costs to run: The single most cost-effective way to fix a green pool, and the most commonly under-dosed. Half a dose twice does not equal a full dose once — it just wastes the bag.
One honest season: The cheapest setup that genuinely works for a season of real swimming. Nothing spare, nothing missing.
Cloudy water that is chemically fine
Pool Water Clarifier
Various
Clumps particles too small for your filter into ones it can catch. Genuinely useful for the specific case where your chemistry reads correct and the water is still milky.
Key specs
- Function
- Coagulant — aggregates fine particles for filtration
- Requires
- A running filter; it does nothing on its own
- Typical treatment
- Dose, then run the filter continuously
How it scoresour reading of published specs, not a test result
- Effectiveness
- 7.5
- Ease
- 8.5
- Water impact
- 8.0
- Cost per season
- 8.5
- Value
- 7.5
Overall 7.8 / 10
What works
- Fixes the exact case where chemistry is balanced and the water still will not clear
- Cheap and quick when it is the right tool
- Works with the filter you already have rather than needing a better one
What doesn't
- Does nothing at all if the actual problem is low chlorine or a blocked filter
- Overdosing makes cloudiness worse, which is a genuinely counterintuitive failure
- Loads the filter, so it needs cleaning right after
Skip it if: Being the first thing you reach for. Test the water before you use this — most cloudy pools are a chemistry problem wearing a clarity costume.
What it costs to run: A few dollars a bottle, and worth it only when the decision tree points here. Reaching for clarifier first is how people spend twenty dollars solving a problem that a longer pump run would have fixed for free.
A few weekends: Fine for a heatwave and a toddler. Treat it as consumable, not as a pool — it will not survive a winter in the yard.
The consumable you will actually buy repeatedly
Type A Replacement Filter Cartridges (multipack)
Various
Buy them in a multipack before you need them. A pool running on an exhausted cartridge is a cloudy pool, and the fix is a two-dollar part you did not have.
Key specs
- Type
- Type A — the most common Intex format
- Replacement interval
- Every 2 weeks in heavy use, per Intex guidance
- Between changes
- Rinse, do not scrub
How it scoresour reading of published specs, not a test result
- Sizing fit
- 9.0
- Running cost
- 7.0
- Maintenance
- 8.5
- Build
- 7.0
- Value
- 8.5
Overall 8.2 / 10
What works
- Type A is the most widely stocked cartridge format in the US
- Multipacks cost a fraction of buying singles
- Having a spare turns a cloudy week into a five-minute fix
What doesn't
- It is a genuine recurring cost that nobody includes in the purchase price
- Aftermarket quality varies noticeably
- Rinsing extends life but does not restore a cartridge that is done
Skip it if: Being restored. Once the pleats are permanently gray and stiff, rinsing is not saving it.
What it costs to run: The line item most people leave out of their pool budget. Count how many you use in your first season and you have your real annual filtration cost — this is exactly the number the cost-to-run guide asks you to record.
One honest season: The cheapest setup that genuinely works for a season of real swimming. Nothing spare, nothing missing.
When you need a number, not a color band
Taylor K-2006 Complete Test Kit (FAS-DPD)
Taylor Technologies
The reference kit. It uses a drop-count titration for free chlorine, which gives you an actual figure rather than a shade of pink. Overkill for a 1,500-gallon pool and indispensable when something has gone properly wrong.
#ad how this is funded · price as of October 8, 2026
Key specs
- Method
- FAS-DPD drop-count titration
- Measures
- Free and combined chlorine, pH, alkalinity, calcium hardness, cyanuric acid
- Resolution
- Reads to 0.2 ppm free chlorine with the standard 10 mL sample
- Format
- Reagent bottles and graduated comparators
How it scoresour reading of published specs, not a test result
- Accuracy
- 9.8
- Ease
- 5.5
- Coverage
- 9.5
- Cost per use
- 7.0
- Value
- 8.0
Overall 8.3 / 10
What works
- Distinguishes free from combined chlorine, which is the single most useful thing a test can tell you when water smells of chlorine
- Gives a numeric result you can act on precisely rather than a color to interpret
- Reagents are individually replaceable, so the kit is a multi-season purchase
What doesn't
- Considerably more expensive than strips and more of a ritual to use
- Reagents have a shelf life and do expire
- Genuinely more kit than a small seasonal pool needs
Skip it if: Casual weekly checks. If it is a five-minute job you will skip it, and a test you skip is worse than a rough one you actually do.
What it costs to run: The reagents are the running cost, not the kit. For most seasonal above-ground owners this is a second purchase made in year two after a bad algae summer, not a first one.
Several seasons: Buy-once. Built and warranted to come back out of the box for a few years, if you store it properly.
Specs from: Taylor Technologies
Questions people actually ask
What does green pool water mean?
Why is my pool water milky white?
How can I tell if the color is in the water or on the liner?
Is it normal for a pool to go cloudy after shocking?
Sources
Read next
- Pool care, from someone who assumes you have never done thisThe weekly routine, the four numbers that matter, and the fix for the week the water turns.
- Cloudy pool water: work out which of four problems you haveA diagnostic decision tree for cloudy water - four causes, how to tell them apart, and the correct fix for each.
- Pool algae: brush, shock, filter, then rebalanceThe correct order for clearing green water, how to tell algae from pollen, and why algaecide prevents rather than cures.
- Mustard algae, and why brushing it away is not the same as killing itHow to recognize mustard algae, why normal chlorine levels do not clear it, and the full treatment including the things that reinfect the pool.
- Stains on the liner, and the one test that tells you which kindOrganic or metal? The five-minute test that identifies a liner stain, plus what removes each kind and what prevents both.
- Clear water and safe water are two different thingsWhy a pool can look perfect and be unsanitized, why haze is often harmless, and the four readings that answer the question clarity cannot.
- Foam on the water is a report on what went inThe four things that make a pool foam, how to tell them apart, and why anti-foam products hide the cause rather than fixing it.
- Filter cartridges: the consumable nobody budgets forThe replacement interval, the rinse-versus-scrub question, and how the cartridge treadmill compares with a sand filter across a season.
- Floc or clarifier: your filter decides, not your waterThe mechanism difference between flocculant and clarifier, why most above-ground pools cannot use floc at all, and when neither is the right answer.