Green water is algae. It means free chlorine dropped too low for your stabilizer level for long enough for algae to bloom.
Likely causes
- Free chlorine below about 7.5% of CYA
- Very high CYA from daily trichlor tablets or dichlor
- Pump not running long enough or a dirty filter
- Heavy rain, heat wave or a big swim party without extra chlorine
The fix, step by step
- Test pH and CYA. Lower pH to about 7.2–7.4.
- Remove debris, empty baskets, clean the filter.
- Raise free chlorine to shock level — about 40% of your CYA.
- Test 2–3 times a day and top chlorine back up to shock level each time.
- Brush daily and run the pump 24/7.
- Stop when the water is crystal clear and chlorine drops 1 ppm or less overnight.
Diagnosis
Cloudy or milky water
Haze usually traces back to low chlorine, high pH or a filter that is not keeping up — in that order.
Likely causes
- Low free chlorine (early algae or bacteria)
- High pH with high calcium or alkalinity (calcium precipitating)
- Dirty, undersized or damaged filter
- Short pump run time
- Fine dust, pollen or new plaster dust
The fix, step by step
- Test free chlorine and pH first; fix those before anything else.
- Check filter pressure; clean or backwash if it is 8–10 psi above clean.
- Make sure the pump gives one full turnover a day.
- Check balance with the LSI calculator.
- For leftover fine haze, use a clarifier and keep filtering.
Diagnosis
Black or dark blue spots
Raised black spots on plaster or in grout are usually black algae — a stubborn cyanobacteria that roots into porous surfaces.
Likely causes
- Rough or porous plaster and grout
- Chlorine that dipped low at some point
- Low-circulation corners and steps
The fix, step by step
- Brush spots hard to break their protective layer (stainless brush on plaster only).
- On plaster, rub spots with a trichlor tablet using gloves.
- Shock to the level your CYA requires and hold it — often a week or more.
- Keep brushing daily until spots stop returning.
Diagnosis
Yellow or mustard dust
Yellow-brown dust on shady walls that brushes off and returns is mustard algae — it hides on toys, swimsuits and equipment.
Likely causes
- Chlorine too low for CYA level
- Reintroduction from swimsuits, floats and pool tools
The fix, step by step
- Shock to the level your CYA requires and hold it.
- Brush the whole pool daily.
- Clean every toy, float, brush and the vacuum hose; wash swimsuits.
- Hold chlorine until it is stable overnight.
Diagnosis
Pink or white slime
Pink or white slime in corners and fittings is a bacterial or fungal biofilm, not algae.
Likely causes
- Low-circulation spots and fittings
- Contaminated filter media
- Chlorine dipping too low
The fix, step by step
- Shock and hold at the right level for your CYA.
- Scrub every fitting, ladder, light niche and skimmer throat.
- Deep-clean or replace filter media if it keeps returning.
Diagnosis
Stinging eyes or strong chlorine smell
That “chlorine smell” is usually chloramines (combined chlorine) — a sign you need more chlorine, not less. pH out of range also stings.
Likely causes
- Combined chlorine above 0.5 ppm
- pH below 7.2 or above 7.8
- Heavy swimmer load, sweat and sunscreen
The fix, step by step
- Test combined chlorine and pH.
- Correct pH into 7.4–7.6.
- If CC is above 0.5 ppm, raise free chlorine to shock level until CC drops.
- Encourage pre-swim showers on busy days.
Diagnosis
Chlorine will not hold
If chlorine vanishes day after day, something is consuming it — usually sunlight without enough stabilizer, or hidden algae.
Likely causes
- CYA too low (sun burns chlorine off)
- Hidden or early algae
- Heavy organic load (leaves, pollen, swimmers)
- Salt cell output too low or failing
The fix, step by step
- Test CYA; bring it to 30–50 ppm (60–80 for salt).
- Do an overnight chlorine loss test: more than 1 ppm lost overnight means something is still consuming it.
- If so, shock and hold like a green pool.
- Check salt cell output and plates.
Diagnosis
pH keeps rising
Constant upward pH drift is normal for many pools — aeration and salt cells drive it. High alkalinity makes it worse.
Likely causes
- Alkalinity on the high side
- Aeration from spillovers, waterfalls and jets
- Salt chlorine generators
- New plaster curing
The fix, step by step
- Lower total alkalinity toward 60–80 ppm.
- Add acid in small, regular doses.
- Reduce unnecessary aeration.
- Consider adding borates (about 50 ppm) to help stabilize pH.
Diagnosis
White crust or scale
White crusty deposits at the waterline, on tile or in the salt cell are calcium scale from over-saturated water.
Likely causes
- High pH
- High calcium hardness and alkalinity
- High water temperature
- Evaporation concentrating minerals
The fix, step by step
- Check balance with the LSI calculator; aim for -0.3 to +0.3.
- Lower pH and alkalinity first.
- Partially drain and refill if calcium is very high.
- Remove existing scale with a pool-safe scale remover or pumice on tile.
Diagnosis
Brown, green or blue stains
Stains come from metals (iron, copper) or organics (leaves, berries). A simple test tells you which.
Likely causes
- Iron or copper from fill water, corroding equipment or copper algaecides
- Leaves and organic debris left on the floor
- Low pH corroding metal heaters and fittings
- Rusting steel behind a breached liner or coating
The fix, step by step
- Rub a chlorine tablet on the stain: if it lightens, it is organic; if not, rub vitamin C (ascorbic acid) — if that lightens it, it is metal.
- Treat organics with shock and brushing.
- Treat metals with an ascorbic acid stain treatment, then a sequestrant.
- Find and fix the metal source — low pH and corroding equipment are common culprits.
Steel and container pools: any breach in the waterproofing lets water reach steel. Why the membrane matters.
Diagnosis
Foam on the surface
Foam usually comes from cheap algaecides, soaps, lotions or high dissolved solids.
Likely causes
- Inexpensive quat-based algaecides (polyquat types do not foam)
- Body lotions, hair products and detergent in swimsuits
- Very high total dissolved solids
The fix, step by step
- Stop using foaming algaecides.
- Shock to break down organics.
- Partially drain and refill if TDS is very high.
Diagnosis
Air in the pump or low flow
Bubbles in the pump basket and weak returns point to a suction-side air leak or a blockage.
Likely causes
- Low water level pulling air through the skimmer
- Cracked or dry pump lid o-ring
- Loose suction fittings or valves
- Clogged baskets, impeller or filter
The fix, step by step
- Top up water to mid-skimmer.
- Clean and lubricate the pump lid o-ring (or replace it).
- Empty skimmer and pump baskets; check the impeller for debris.
- Clean the filter; call a technician for cracked plumbing or motor issues.
Some loss is evaporation. The bucket test tells you in 24 hours whether you have a real leak.
Likely causes
- Evaporation (hot, dry, windy weather)
- Skimmer, light niche or fitting leaks
- Plumbing leaks at the pad or underground
- Cracks in the shell or a breached liner or coating
The fix, step by step
- Run the bucket test for 24 hours.
- Repeat with the pump off to separate plumbing from shell leaks.
- Use dye at suspect spots.
- Call a leak detection pro if you cannot find it.
Steel and container pools: any breach in the waterproofing lets water reach steel. Why the membrane matters.
Diagnosis
Rough, peeling or blistering surface
Rough, etched, peeling or blistering surfaces mean the finish is wearing out or was never bonded properly.
Likely causes
- Aggressive (low pH / low calcium) water etching plaster
- Paint at the end of its life
- Poor surface prep or moisture under a coating
- Age and freeze-thaw cycles
The fix, step by step
- Balance water to stop further damage (LSI near zero).
- Document the damage with photos.
- Compare resurfacing options for your pool type.
- For steel or container pools, have breached areas assessed quickly to stop corrosion.
Steel and container pools: any breach in the waterproofing lets water reach steel. Why the membrane matters.