Master pool care cheat sheet →
🧭Troubleshooting cheat sheet
Pool Chemical Problems Chart: What Chemical Fixes What
Find your symptom, see the cause, use the right lever, dose it exactly.
Updated · Reviewed by the Skimmer editorial team
Fix order
- 1Total alkalinity first: it steadies pH
- 2Then pH
- 3Then free chlorine (shock if CC is over 0.5)
- 4Then CYA and calcium, which change slowly
- 5Pump running, one chemical at a time, retest after 4 to 6 hours
What did you measure?
Enter any readings you have. You get the fixes in the order to do them.
What do you see?
| You see / measure | Likely cause | The lever | Target | Don't |
|---|---|---|---|---|
| Green water | Algae from low free chlorine | SLAM: raise FC to 40% of CYA and hold it, brush, filter 24/7 SLAM plan | FC at SLAM level until clear | Dump algaecide instead of chlorine |
| Yellow or mustard dust on walls | Mustard algae (brushes off) or iron (does not) | Algae: SLAM and wash every tool. Metals: sequestrant Shock dose | FC at SLAM level | Assume it is pollen; brush it first to tell |
| Brown or rust-colored water | Iron, often from well water or corroded parts | Metal sequestrant, then filter and clean the filter | Iron 0 ppm | Shock first; chlorine oxidizes iron and sets stains |
| Blue-green after shocking | Copper from algaecide, ionizers or a low-pH heater | Sequestrant, raise pH to 7.4 to 7.6, stop copper algaecide pH dose | pH 7.4 to 7.6, copper 0 | Shock again; it is metal, not algae |
| Milky cloudy water | Low FC, poor filtration, or high pH and calcium | Raise FC, check pH and TA, clean the filter, run 24/7 Cloudy water guide | FC in range, pH 7.4 to 7.6 | Add flocculant before fixing FC |
| Cloudy right after shock | Dead algae and debris the filter has not caught yet | Keep FC up, run the filter, clean it when pressure rises Cloudy after shock | Filter psi back near clean baseline | Stop shocking because it looks worse |
| Black spots that won't brush off | Black algae rooted in plaster | Steel brush daily, SLAM, rub a trichlor tab on spots (plaster only) Black vs green algae | FC at SLAM level for 1 to 2 weeks | Use a steel brush on vinyl |
| Pink or white slime in corners | Bacterial biofilm (often called pink or white mold) | Brush, SLAM, clean or replace hoses and toys | FC at SLAM level | Skip cleaning the skimmer and fittings |
| Foam on the surface | Cheap algaecide, lotions or soft water | Switch to a polyquat algaecide, run the filter, partial drain if it persists | No foam | Add more algaecide |
| pH too high | Normal drift, aeration, new plaster, high TA | Muriatic acid or dry acid pH calculator | pH 7.4 to 7.6 | Add acid and chlorine at the same spot at once |
| pH too low | Acid overdose, trichlor tabs, rain | Soda ash pH calculator | pH 7.4 to 7.6 | Use baking soda for a big pH jump; it mostly raises TA |
| Alkalinity high | Fill water, too much baking soda | Acid, then aerate to bring pH back up TA calculator | TA 60 to 120 ppm | Expect it to drop in one dose |
| Alkalinity low | Acid use, trichlor, rain dilution | Baking soda TA calculator | TA 60 to 120 ppm | Use soda ash; it overshoots pH |
| Free chlorine low or won't hold | Algae starting, low CYA, or high demand | Raise FC; if it drops more than 1 ppm overnight, SLAM Chlorine calculator | FC about 7.5% of CYA or more | Blame chlorine lock before testing CYA |
| Combined chlorine over 0.5 ppm | Chloramines from sweat and urine (the chlorine smell) | Shock to breakpoint Shock dose | CC under 0.5 ppm | Assume the smell means too much chlorine |
| CYA too high | Years of trichlor or dichlor | Partial drain and refill (nothing else lowers it reliably) CYA calculator | CYA 30 to 50 (60 to 80 salt) | Keep using stabilized tabs |
| CYA too low | Fresh fill, splash-out, liquid chlorine only | Stabilizer (cyanuric acid) in a sock in the skimmer CYA calculator | CYA 30 to 50 | Backwash for 2 days after adding it |
| Calcium hardness high | Hard fill water, evaporation, cal-hypo | Partial drain and refill Calcium calculator | CH 200 to 400 ppm | Buy a 'hardness reducer'; most only mask scale |
| Calcium hardness low | Soft fill water, rain | Calcium chloride Calcium calculator | CH 200 to 400 ppm (plaster) | Ignore it on plaster; it etches |
| Salt low (salt pools) | Splash-out, backwash, rain | Pool salt Salt calculator | Salt 2,700 to 3,400 ppm | Trust the cell reading without a test strip |
| Phosphates high | Fertilizer, leaves, some fill water | Keep FC in range; phosphate remover is optional Chlorine calculator | Under 1,000 ppb | Treat phosphates as the cause of algae |
| Stinging eyes or itchy skin | pH off, or combined chlorine high | Fix pH first, then shock if CC is over 0.5 pH calculator | pH 7.4 to 7.6, CC under 0.5 | Lower chlorine to fix it |
| Scale on tile or white crust | High LSI: high pH, TA or calcium | Lower pH and TA; check LSI LSI calculator | LSI -0.3 to +0.3 | Scrub with metal on glass tile |
| Etching or rough plaster | Low LSI: low calcium, pH or TA | Raise calcium, then pH and TA LSI calculator | LSI -0.3 to +0.3 | Let a low LSI run all season |
| Stains on steps or floor | Metals (brown, blue-green) or leaves (brown, organic) | Rub a vitamin C tablet on it: if it lifts, it is metal; use a sequestrant | Metals 0 ppm | Shock a metal stain; it sets it |
Chemical to what it moves
| Chemical | Raises | Lowers | Side effects |
|---|---|---|---|
| Muriatic acid | pH, TA | Fumes; add slowly in front of a return | |
| Dry acid (sodium bisulfate) | pH, TA | Adds sulfates over time | |
| Soda ash | pH (fast), TA | Can cloud water if overdosed | |
| Baking soda | TA (mostly), pH slightly | Gentle; good for TA | |
| Liquid chlorine | FC | Raises pH a little, then it drifts back | |
| Cal-hypo | FC | Adds calcium (about 0.7 ppm per 1 ppm FC) | |
| Trichlor / dichlor | FC | pH (trichlor) | Adds CYA every dose |
| Cyanuric acid | CYA | pH slightly | Only a drain removes it |
| Calcium chloride | CH | Gets hot when it dissolves | |
| Pool salt | Salt | Only for salt cell pools | |
| Borates | Borates | Buffer pH drift | |
| Clarifier | Cloudiness | Helps the filter catch fine particles | |
| Flocculant | Cloudiness | Must vacuum to waste after | |
| Metal sequestrant | Visible metals | Needs topping up; does not remove metals |
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General reference for residential pools. Product labels and local guidance override any figure here. See our methodology and editorial policy. Skimmer · skimmer pool care.
Frequently asked
What fixes what: questions
Why is my pool cloudy after shocking?
Dead algae and debris are suspended in the water and the filter has not caught them yet. Keep chlorine up, run the filter around the clock and clean it when the pressure rises. It usually clears in 1 to 3 days.
Can I add chlorine and pH down at the same time?
No. Add one, let the pump circulate it for at least 30 minutes to a few hours, then add the other in a different spot. Chlorine and acid mixed together release chlorine gas.
What do I fix first?
Total alkalinity, then pH, then free chlorine, then CYA and calcium. Alkalinity holds pH steady, and chlorine works best at the right pH.
Why won't my chlorine hold?
Usually algae you cannot see yet, CYA too low (sun burns chlorine off) or very high demand. Do an overnight chlorine loss test: if FC drops more than 1 ppm overnight, SLAM the pool.
Is blue-green water after shocking algae or copper?
If it turned blue-green right after a shock and the water is clear, it is usually copper oxidized by the chlorine. Add a sequestrant and bring pH to 7.4 to 7.6 instead of shocking again.
How long do I wait to swim after adding chemicals?
After balancers like acid or baking soda, about 20 to 30 minutes with the pump on. After shocking, wait until free chlorine is back under about 10 ppm.