Understanding Pool Water: pH, Conductivity, TDS, Chlorine, UV, Algae, and the Right Water Source

Anyone running a private pool quickly realises: the biggest topic isn’t the pump or the filter — it’s the water chemistry. Understanding what pH, conductivity, TDS, and water hardness actually mean, where these terms come from, and how to measure them reliably saves you from algae-infested water, irritated skin, corroded equipment, and expensive chemical mistakes. This article brings order to the key concepts — explained thoroughly and from first principles, with concrete practical reference values.

 

1. Water and the pH Level

What the pH value measures

The pH value describes how acidic or basic (alkaline) an aqueous solution is — on a scale from 0 (strongly acidic) to 14 (strongly basic), with 7 being neutral. Target range in a pool: 7.0–7.4 (slightly on the alkaline side, close to the pH of tears and mucous membranes).

Where does the term “pH” come from — and how was it discovered?

The concept of pH is not some ancient natural constant — it’s a comparatively young, very practically motivated invention. The concept was introduced in 1909 by Søren Sørensen as a convenient way to express acidity — as the negative logarithm of hydrogen ion concentration. Sørensen (1868–1939) directed the chemistry department of the Carlsberg Laboratory, which was funded by the brewery of the same name — brewing being one of the oldest chemical industries in existence.

That’s no coincidence: Sørensen developed the pH scale while investigating the effect of ion concentration on proteins at the Carlsberg Laboratory, at a time when the field was shifting from colour-based acidity tests toward electrical measurement methods. He needed a precise, reproducible measure of hydrogen ion concentration for quality control of enzymes used in the brewing process — the old colour-indicator tests were too imprecise and too subjective for that.

The real motivation: hydrogen ion concentrations in aqueous solutions are tiny — often in the range of 0.0000001 mol/l or smaller. Rather than working with such unwieldy powers of ten, Sørensen defined the pH value as the negative base-10 logarithm of that concentration:

pH = −log₁₀[H⁺]

A hydrogen ion concentration of 10⁻⁷ mol/l — the value at which pure water sits at 25 °C — thereby becomes simply “pH 7”. Each full step on the scale represents a tenfold increase or decrease in hydrogen ion concentration — pH 6 is ten times more acidic than pH 7, pH 5 a hundred times more acidic.

An interesting side note: exactly where the letters “p” and “H” came from is not fully and unambiguously documented, even among historians of science. The idea of expressing hydrogen ion concentration on a logarithmic scale was presented by Sørensen in 1909; the symbol he used was the letter p with a smaller H that appeared almost as a subscript. What is established is only this: the “H” stands for hydrogenium (hydrogen), and the “p” marks the negative exponent (the “power”) of 10 — in the sense of “the power of ten used to express the concentration”.

Sørensen built on earlier work: the foundation was Svante Arrhenius’s earlier work, whose 1884 definition of an acid was that it dissociates in solution to produce hydrogen ions. In order to determine the hydrogen ion concentration precisely, without resorting to colour tests, Sørensen devised an experiment in which the concentration gradient of the ions could be related to the electrical gradient between electrodes in an electrochemical cell. In effect, he measured a voltage between two electrodes (a hydrogen electrode and a reference electrode) and used that — via what is today known as the Nernst equation — to calculate back to the hydrogen ion concentration.

This electrochemical measurement method remains, to this day, the operating principle of modern digital pH meters: a glass electrode generates a small voltage that depends on the hydrogen ion concentration, which the instrument converts into a pH reading. The old colour-indicator tests (litmus, phenol red, etc.), which today’s test strips are still based on, are essentially the “predecessor technology” that Sørensen set out to replace.

Why the pH value matters so much in a pool

The pH value is closely tied to Total Alkalinity (TA) — the water’s buffering capacity against pH swings. Target TA: 80–120 mg/l (as CaCO₃). If TA is too low, the pH value “swings” strongly with every small addition — you end up chasing it constantly.

 

2. What Is Conductivity?

Definition

Electrical conductivity (EC, unit µS/cm or mS/cm) measures how well water conducts electrical current. Pure water (H₂O) barely conducts at all — conductivity only arises from dissolved ions (salts, minerals, chloride, sulphate, calcium, sodium, etc.).

Rule of thumb: the more dissolved substances in the water, the higher the conductivity.

Where does the unit “Siemens” come from?

Here, too, there’s a concrete historical story — and fittingly, it involves a German industrial pioneer. The Siemens unit, named after the German electrical engineer Werner von Siemens, was a unit introduced in German-speaking countries in 1860 for determining electrical resistance. It was defined as the electrical resistance occurring in a mercury column 1 m long with a cross-section of 1 mm² at a temperature of 0 °C. This early “Siemens unit” corresponded to roughly 0.94 ohms, and was an early attempt to create a globally reproducible resistance standard — before that, virtually every country, or even every laboratory, had its own measure.

Today’s SI unit “siemens” (symbol S) is not identical to this original 1860 mercury-based unit, but rather a later honour: in 1935 the International Electrotechnical Commission introduced the siemens as a unit, and at the 14th General Conference on Weights and Measures in 1971 the siemens was officially adopted into the SI system. Since then, siemens denotes electrical conductance — the reciprocal of electrical resistance (ohm):

1 siemens (S) = 1 / ohm = 1 ampere / volt

Werner von Siemens (1816–1892) founded the industrial company that bears his name and was one of the most influential electrical engineers of the 19th century (the pointer telegraph, the dynamo-electric machine, the first electric locomotive). That the unit for electrical conductivity is named after him specifically honours his pioneering work in standardising reproducible electrical measurements — precisely the problem that led him to his resistance standard in 1860.

For water analysis, the area-related units µS/cm (microsiemens per centimetre) or mS/cm (millisiemens per centimetre) are typically used, since conductivity depends strongly on the geometry of the measuring cell, and these units make readings comparable.

Why conductivity matters so much

Conductivity is such a useful measurement because it delivers, within seconds — with no chemicals, no reagents — a reliable indication of the total mineral or salt content of a water sample. It’s used, among other things, for:

• assessing drinking and process water quality,

• monitoring reverse osmosis and desalination plants (if conductivity rises in the permeate, membrane performance is declining),

• irrigation in agriculture (excessive conductivity in irrigation water can damage roots and salinate soils),

• and, of course, pool water maintenance — especially with salt-water electrolysis systems.

Reference values for orientation:

• Distilled/reverse-osmosis water: ~1–10 µS/cm

• Rainwater: ~5–30 µS/cm

• Tap (drinking) water: ~200–800 µS/cm

• Pool water (normal, with salt/chlorine additives): ~1,000–3,000 µS/cm

• Salt-water pool (electrolysis): often 3,000–6,000 µS/cm, depending on salt content

The conductivity of seawater (Atlantic example)

Seawater sits in a completely different order of magnitude from freshwater. Freshwater usually ranges between 0 and 1,500 µS/cm, while typical seawater has a conductivity value of around 50,000 µS/cm. More precisely: due to its high salt content, the electrical conductivity of seawater is significantly higher than that of freshwater, typically 50–60 mS/cm.

For the Atlantic specifically, this means: with a typical open-Atlantic salinity of around 35–37 ‰ (in the subtropical North Atlantic, where strong evaporation occurs, tending toward the upper end of that range), surface conductivity sits at around 50,000–55,000 µS/cm (50–55 mS/cm), depending on water temperature — conductivity rises with temperature, which is why warm tropical/subtropical surface layers show somewhat higher values than cold deep water. Seawater with a salinity of 35 ‰ typically has a conductivity of about 53 mS/cm.

For comparison with a pool: even a heavily salted salt-water pool (electrolysis system, roughly 3,000–6,000 µS/cm) reaches only about a tenth to a fifteenth of the conductivity of actual seawater — a pool “tastes” salty, but chemically it’s far from a true seawater concentration.

 

3. What Is TDS?

TDS = Total Dissolved Solids, measured in mg/l or ppm. It’s an aggregate value for all inorganic and organic substances dissolved in water: salts, minerals, metals, but also organic residues from sunscreen, sweat, cosmetics, etc.

TDS and conductivity are related but not the same:

• Conductivity directly measures the ability to conduct current.

• TDS is usually calculated from conductivity (conversion factor depending on the device, roughly 0.5–0.7 × µS/cm = ppm TDS).

Reference values for pools:

• < 1,000 ppm: unproblematic

• 1,000–1,500 ppm: still acceptable, but worth watching

• > 1,500–2,000 ppm (for a chlorine-based, non-salt-water pool): a partial water change is advisable — high TDS values encourage cloudy water, reduced chlorine effectiveness, odour formation, and deposits

Important: In salt-water pools, TDS is naturally much higher (due to the added salt itself), so the threshold above doesn’t apply on a 1:1 basis.

4. How Can I Reliably Measure pH and Conductivity? Test Strips or a Meter?

In short: test strips are fine for a quick check; reliable values require a meter.

Practical recommendation for a private pool: - Test strips for daily quick checks (usually sufficient for chlorine + pH trend). - A digital, calibratable pH meter (e.g. with ATC = automatic temperature compensation) for reliable values, especially before dosing chemicals. - A simple TDS/EC pen meter (10–30 €) is enough for salt-water pools or when feeding in groundwater/rainwater and wanting to keep an eye on mineralisation. - Calibration is the crux of the matter: an uncalibrated digital device can be worse than a good test strip. Use buffer solutions (pH 7 and 10) every few weeks, or before important measurements.

 

5. Measuring Chlorine

With chlorine, three values are distinguished:

• Free chlorine (FC): the actively effective, disinfecting chlorine. Target range: 1–3 mg/l (private pool); with salt-water pools, the lower end is often sufficient.

• Combined chlorine (chloramines, CC): forms through the reaction of chlorine with ammonia/organic substances (sweat, urine, cosmetics). Should be < 0.2 mg/l — high values are the typical “indoor pool smell”, cause eye irritation, and indicate that there’s actually too little, not too much, chlorine in the water.

• Total chlorine (TC) = free chlorine + combined chlorine.

Measurement methods:

• DPD test (tablets or drops): the standard method, colour-based, distinguishing FC and TC (via two reagents, DPD1 and DPD3). Read with a simple colour-comparison block (“pool tester”) or evaluated photometrically.

• Test strips: fast but imprecise, usually only total chlorine or coarse FC steps.

• Digital photometers: give precise mg/l values for FC and CC separately — recommended if you’re adjusting frequently or dealing with odour/irritation issues.

Practical tip: chlorine readings are highly sensitive to light (UV breaks down free chlorine) and temperature — it’s best to measure in the morning, before strong sunlight and before swimming.

 

6. UV Disinfection in the Pool

Alongside chlorine, UV disinfection is gaining importance as a complementary (not replacement) method — particularly for private pools, where less chlorine odour and gentler water quality are desired.

How it works

UV disinfection units irradiate the pool water flowing through them with UV-C light in the wavelength range of roughly 200–280 nm, with peak effectiveness around 254 nm. This radiation penetrates the cells of bacteria, viruses, algae, and single-celled organisms, damaging their DNA/RNA there (more precisely: it causes the formation of so-called thymine dimers), so the microorganisms can no longer reproduce. This is therefore a purely physical disinfection method, without any additional chemicals.

Two designs

• Low-pressure UV lamps: produce nearly monochromatic light at 254 nm, are energy-efficient, and are optimised purely for killing pathogens.

• Medium-pressure UV lamps: emit a broader spectrum (roughly 200–400 nm) at higher power. They can additionally break down chloramines (combined chlorine) photochemically — an important extra benefit for pools, since this tackles odour formation and eye irritation at the root (see Chapter 5).

Advantages

• Effective even against chlorine-resistant pathogens (e.g. Cryptosporidium cysts), which survive even normal chlorine concentrations.

• Noticeably reduces the required chlorine dose (in practice, often 30–50% less chlorine consumption with combined systems).

• Breaks down combined chlorine (chloramines) — less odour, less irritation of eyes and mucous membranes.

• No additional chemicals, no effect on pH value or water hardness.

Limitations — why UV doesn’t fully replace chlorine

The decisive drawback: UV light has no residual effect. Unlike chlorine, which stays dissolved throughout the pool water and works continuously there, UV only takes effect exactly at the moment the water passes the lamp. As soon as the disinfected water flows back into the pool, it’s unprotected against newly introduced germs (from swimmers, leaves, insects, rainwater). This is why a base concentration of chlorine or bromine remains necessary as “residual protection” in the pool — UV complements chemical disinfection but doesn’t replace it entirely.

Further practical points:

• Effectiveness depends on water clarity — cloudy or heavily contaminated water lets less UV radiation through (the keyword being turbidity).

• UV lamps age and lose power; replacement is typically needed roughly every 9,000–12,000 operating hours, or annually, depending on the model.

• The quartz glass sleeve around the lamp can scale up (see Chapter 7, water hardness) and must be cleaned regularly, as limescale deposits significantly reduce UV transmission.

 

7. Water Hardness — Overview and Chart

Water hardness describes the calcium and magnesium ion content of water. It’s expressed in various units:

Recommended range for pools: 8–18 °dH (approx. 150–250 ppm calcium hardness).

• Water that’s too soft (< 8 °dH): aggressive — attacks concrete, grout, metal parts, and pool liners (“hungry water” draws calcium from its surroundings).

• Water that’s too hard (> 18–20 °dH): limescale deposits at the pool edge, cloudiness, scaling of heating elements, the filter system, and — as mentioned above — the UV quartz sleeve.

Water hardness can be determined with test strips (roughly) or drop tests (more accurate, EDTA titration). Digital hardness meters are rarely necessary for private use.

 

8. Water — Chemical Composition Explained

Pure water (H₂O) never occurs in its pure form in nature. Every water source carries dissolved substances, which can broadly be grouped into categories:

• Cations: calcium (Ca²⁺), magnesium (Mg²⁺), sodium (Na⁺), potassium (K⁺)

• Anions: bicarbonate (HCO₃⁻, determines alkalinity), sulphate (SO₄²⁻), chloride (Cl⁻), nitrate (NO₃⁻)

• Dissolved gases: CO₂ (affects pH), O₂

• Organic substances: humic substances (especially in surface/rainwater), sweat, cosmetic residues, pollen (in pools)

• Microorganisms: algae, bacteria — controlled in pools through disinfection (chlorine, UV, ozone)

This composition directly determines:

• conductivity (more ions = higher conductivity),

• the TDS value (sum of all dissolved substances),

• hardness (Ca²⁺/Mg²⁺ content),

• buffering capacity/alkalinity (HCO₃⁻ content), and therefore pH stability.

You can picture this like a system of connected pipes: change one value (e.g. by diluting with rainwater), and the others automatically shift too.

 

9. pH Correction

Once the pH value has been measured (see Chapter 4), the question becomes: how do you correct it cleanly, without over-correcting the water or damaging the structure/equipment?

Lowering pH (value too high, > 7.4)

Most commonly used:

• Sodium bisulphate (NaHSO₄), usually sold as “pH minus” granules: solid, easy to dose, relatively mild to handle.

• Diluted hydrochloric acid (HCl), sold as “liquid pH minus”: acts faster, but is more corrosive to handle and must be dosed more carefully.
  
  

Raising pH (value too low, < 7.0)

• Sodium carbonate (soda ash, Na₂CO₃), as “pH plus” granules: reliably raises pH, while also slightly raising alkalinity (TA).

• Sodium bicarbonate (baking soda, NaHCO₃): acts mainly on alkalinity and only moderately on pH directly — well suited when it’s primarily TA (see Chapter 1) that’s too low.
  

The correct order

1. First check and adjust alkalinity (TA) (target range 80–120 mg/l). It’s the buffer that keeps pH stable — ignore it, and any pH correction “swings” unnecessarily strongly.

2. Only then correct the pH value.

3. Never add multiple chemicals at once or undiluted directly into the pool — in particular, never mix or introduce acid and chlorine products together (risk of toxic gases, e.g. chlorine gas).

Practical procedure

• Calculate the amount according to product instructions and pool volume (never estimate “by feel” — the required amount also depends on alkalinity: higher TA = more buffer = more product needed to move the pH at all).

• Dissolve granules in a bucket of water beforehand; never scatter them dry into the pool.

• With the circulation pump running, introduce the product slowly and distributed around the pool edge (never concentrated at a single spot — localised over-acidification can attack liner, tiles, or grout).

• Let it circulate for 2–4 hours before re-testing.

• Prefer correcting in several small steps rather than adding the full calculated amount at once — this avoids overshooting in the opposite direction.

• Use protective equipment (gloves, and goggles if appropriate), especially with liquid hydrochloric acid.

 

10. Removing Algae

Algae are the most common visible problem in private pools — usually a sign that something in the chain of pH, chlorine, or circulation has gone wrong somewhere, rather than being the root cause itself.

The main types of algae

Causes

• Free chlorine too low for an extended period (see Chapter 5).

• pH value outside the target range — reducing chlorine effectiveness as a result (see Chapter 1).

• Insufficient filter runtime or a clogged filter (poor circulation = dead zones where algae take hold).

• Nutrient input: phosphates and nitrates from rainwater, leaves, garden fertiliser nearby, bird droppings.

• Warm temperatures and strong sunlight further encourage growth.

Prevention

• Keep pH (7.0–7.4) and free chlorine (1–3 mg/l) constantly within the target range — the single most effective measure.

• Backwash/clean the filter regularly, and ensure sufficient daily circulation time.

• Use a pool cover when not in use — this reduces both external nutrient input and UV-driven chlorine breakdown (see Chapter 1: chlorine is broken down by UV light).

• Preventive, low-dose algaecides (e.g. based on polyquat or copper compounds) can be used as a supplement, but don’t replace correct chlorine/pH management.

Acute treatment for visible algae infestation

1. First adjust pH to 7.0–7.2 (see Chapter 9) — chlorine is most effective in this range.

2. Brush mechanically, especially for yellow and black algae: loosen deposits in grout lines, corners, and rough surfaces with a stiff brush (a wire brush for black algae if needed), so the chemistry can actually reach the algae.

3. Shock chlorination: raise free chlorine to roughly 5 to 10 times the normal value (chlorine granules or liquid chlorine, dosed according to manufacturer instructions for pool size). Best done in the evening or at night so UV light doesn’t immediately break the chlorine down again. Keep the circulation pump running continuously throughout.

4. For stubborn yellow or black algae, chlorine alone is often not enough — add a dedicated algaecide, following the manufacturer’s dosing guidance to avoid discolouration (e.g. from copper if the pH is off).

5. Backwash/clean the filter repeatedly during and after treatment — otherwise dead algae debris clogs the filter and makes the water look cloudy even though the algae are already dead.

6. Re-test and repeat if necessary, until the water is clear and chlorine/pH values are back within the target range.

Practical tip: after shock chlorination, TDS and conductivity rise noticeably (see Chapters 2 and 3) — with repeated algae problems over the course of a season, a partial water change eventually pays off, rather than continuing to dose more chemicals.

 

11. Circulation and Runtime

All the previous chapters — pH, chlorine, UV, algae control — fundamentally assume one thing that’s rarely explained explicitly: that the water is actually moving. Without adequate circulation, even the best water chemistry does little good, because it simply doesn’t reach where it’s needed. Circulation is therefore not a side issue of pool technology, but the basic precondition for every other chapter in this article to actually work in practice.

What circulation actually means

Circulation describes the continuous cycle: water is drawn in through the skimmer and/or main drain, pushed through the filter (sand, cartridge, or diatomaceous earth filter), optionally routed through a heater, UV unit, or dosing station, and returned to the pool via the return jets. The central metric here is the turnover rate: the time needed to send the entire pool volume once, completely, through the filtration circuit.

Turnover time (hours) = Pool volume (litres) ÷ Pump flow rate (litres/hour)

Example: a 50 m³ pool with a pump delivering 10 m³/h requires 5 hours for a complete turnover of the entire water volume.

Why circulation is so crucial

1. Distributing the water chemistry Chlorine, pH correction agents, or algaecides only work where they actually reach. Without circulation, added chlorine stays locally concentrated at the point where it was introduced, while the rest of the pool — corners in particular, steps, the area behind the ladder, or shallow zones — remains effectively untreated. It’s precisely in such “dead zones” with poor water exchange that algae and biofilms preferentially begin to grow (see Chapter 10) — not because less chlorine was added there, but because the chlorine that was added never reached them.

2. Filtration needs flow A filter can only remove suspended particles, skin flakes, sunscreen residue, pollen, and microorganisms from water that actually flows through it. If the pump is running but flow within the pool is weak due to poorly placed jets or too low a flow rate, large volumes of water remain effectively unfiltered in the pool — the filter is technically working, but missing the actual problem.

3. Preventing stratification Without adequate mixing — especially in larger or irregularly shaped pools — temperature and chemical layers can form: warmer, lower-chlorine water on top, cooler, denser water below. This leads to inconsistent readings depending on where you measure, and to areas that remain under-treated despite “correct” average values.

4. A precondition for auxiliary systems UV units (Chapter 6) and heaters only deliver their intended effect if flow through the respective chamber is sufficient — but not excessive. If water flows too quickly through the UV chamber, exposure time per water volume drops, and with it disinfection performance; if it flows too slowly, overall throughput drops, and a larger share of the pool volume goes untreated per unit of time.

5. Mechanical cleaning as a side effect A well-designed flow layout (return jets aligned so that a circulating flow forms across the entire pool surface) actively carries dirt particles toward the skimmer and main drain, rather than leaving them to settle in corners. Circulation is therefore also, in part, a form of mechanical cleaning that works without any chemicals at all.

How long should the pump run?

A rule of thumb commonly used in Germany for daily filtration runtime is based on water temperature:

Filtration runtime (hours/day) ≈ Water temperature (°C) ÷ 2

The reasoning: higher temperatures encourage both algae growth and chlorine breakdown, and, with more swimming activity, increase the input of organic contamination — all of which calls for more circulation, not less.

Practical recommendations

• Spread runtime across the day rather than running it in one block: several shorter intervals (e.g. morning, midday, evening) distribute chemistry and filter performance more evenly than running the entire daily total in a single overnight session.

• Keep it running during the day, particularly during the sunniest hours: that’s exactly when chlorine consumption from UV breakdown and algae growth is highest — circulation shouldn’t run only at night just because it’s cheaper or quieter then.

• Prefer variable-speed pumps: continuous operation at low speed avoids dead zones more reliably than short high-load bursts — and since a pump’s power draw falls with the cube of its speed, continuous operation at reduced speed is often even more energy-efficient than brief full-speed running.

• Backwash/clean the filter regularly: a clogged filter reduces actual flow even while the pump keeps running unchanged — silently extending the turnover time.

• Temporarily increase runtime after shock chlorination or algae treatment (Chapter 10), so that dead material is reliably filtered out of the water rather than settling.

 

12. Water Sources and Their Treatment

Groundwater

Water from wells/boreholes. Composition depends heavily on the rock it seeps through (lime-rich soils → hard water; granitic/silicate soils → softer water). Often mineral-rich, sometimes also containing iron, manganese, or nitrate (agriculture) — ideally test for hardness, iron/manganese (discolouration risk!), and nitrate before using it for a pool.

Natural springs

Water that emerges naturally at the surface, usually filtered through rock/soil. Mineral content and hardness vary greatly depending on the source rock — from very soft (springs from granite/shale) to very hard (karst springs, limestone).

Desalination

Processes for removing salt from seawater or brackish water — mostly via:

• Thermal processes (evaporation/distillation, energy-intensive, common especially in the Middle East)

• Membrane processes (see reverse osmosis)

The result is very pure, low-mineral water — well suited as a starting point for pools, but it may need to be “re-hardened” with minerals (otherwise pure water is corrosive, see Chapter 7).

Reverse Osmosis (RO)

A membrane process in which water is forced under pressure through a semi-permeable membrane that retains ions, salts, and most dissolved substances. Result: very low conductivity (often < 10 µS/cm) and TDS (< 10 ppm).

• Advantage: an excellent starting base, with no unwanted minerals/contaminants.

• Disadvantage for pools: the water is essentially “empty” — with no buffering capacity at all. It must be conditioned with alkalinity/hardness builders before filling a pool, or it will be strongly corrosive and pH-unstable.

Rainwater

Naturally very soft (low hardness, low conductivity ~5–30 µS/cm), slightly acidic (pH often 5.5–6.5 due to dissolved CO₂, and also SO₂/NOx near industrial areas). Attractive for pools as a free, soft water source, but:

• has virtually no alkalinity/buffering capacity → pH tips easily,

• can bring in organic contamination (leaves, bird droppings, roofing material residue) if not collected and filtered cleanly via roof/cistern,

• should be filtered before filling the pool, with alkalinity actively raised (e.g. with sodium bicarbonate).

 

13. Water in the Pool — Putting It All Into Practice

In summary, all the parameters covered above come together in the pool and influence one another:

Practical sequence for water maintenance: 1. Check the source water — whether tap water, groundwater, rainwater, or RO water: measure hardness, alkalinity, and conductivity first, before filling. 2. Basic setup: adjust alkalinity first (it’s the buffer for pH), then pH, then hardness, then disinfection (chlorine/salt electrolysis/UV). 3. Ongoing monitoring: pH and free chlorine daily to every 2–3 days (test strips are fine for trend-checking, a digital meter for precise dosing), TDS/conductivity and hardness monthly or whenever visible issues appear (cloudiness, deposits, odour). 4. Partial water changes: when TDS/conductivity keeps rising (concentration through evaporation — water evaporates, dissolved substances stay behind and concentrate), only a partial water exchange helps; no chemical can undo this.

The basic rule that ties it all together: alkalinity stabilises the pH value, the pH value determines how effectively chlorine works, and hardness protects the physical structure. Get these three under control, and you’ve got the pool under control — everything else is fine-tuning.