Know Your PoolPool water chemistry

A correctly balanced and sanitised pool is important for health and safety reasons, as well as for protecting your pool shell and pool equipment from damage caused by acidic or alkaline water.

Damage caused to your pool shell and equipment by failure to maintain correct water chemistry (in particular damage caused by excess chlorine and pH levels) may void your pool shell and equipment warranties.

Note: For more advice on maintaining a clean and healthy pool and maintaining correct water chemistry, consult your local pool shop.

1. Glossary of terms to help you maintain clean, healthy, and sparkling pool water

Disinfection/Sanitation – Disinfection/sanitation is the process used to kill all the disease-causing organisms in the pool water. The active forms of chlorine and bromine gases, dissolved in the water, are the two most common types of disinfectants.

A measurable active chlorine residual (or approved alternative) must be present in the water to perform satisfactory sanitisation and disinfection.

Oxidisation – Oxidisation is used to destroy organic contaminants such as algae, body oils, minerals and other materials which cause irritation, colour, odour and turbidity. Oxidisation will rid ammonia and nitrogen compounds from pool water. The strength of Oxidisation is measured in millivolts and called ORP (Oxidation Reduction Potential).

Filtration – Filtration is the physical operation of removing undissolved and suspended particles (dirt, debris, oil, skin, hair, organisms) from your pool water. Impurities are trapped as water passes through your filter and clean water is returned to your pool.

Correct and sufficient pump running times are required for satisfactory filtration and sanitisation. Insufficient sanitisation, and cloudy water will result from insufficient running times.

2. Practical Water Chemistry for Fibreglass Swimming Pools

(a) Regular Water Testing

  • After pool safety, the treatment of water is the most important factor of pool ownership.
  • Correct water chemistry will ensure a long service life from your pool and equipment, as well as minimising the risk of disease to pool users from inadequate pool water treatment.
  • Correct disinfection of swimming pool and spa pool water is essential for protecting your health.
  • The main tests to perform regularly with a test kit are pH, Chlorine, and Total Alkalinity. These tests should be done every week or two.
  • Salt, Phosphates, Calcium hardness and Stabiliser (Cyanuric Acid) will also need to be tested and maintained.
  • These water chemistry levels are slower to change and should only require testing every month or two, by your pool professional, to ensure they are maintained at desired levels.
  • Correctly maintaining your pool water chemistry, by regular testing and adjustments, will save you money in the long run, by reducing chemical usage and maximising the life of your pool and equipment
pH. Ideal range = 7.2 - 7.6 ppm (parts per million)
T.A. – Total Alkalinity Ideal range = 100 to 150 ppm
Chlorine Ideal level = 1.0 to 2.0 ppm but up to 3.0ppm is acceptable
Stabiliser (cyanuric acid) Ideal level = 50 ppm
Salt (for salt chlorinated pools) Ideal level = 3000 - 4000 ppm (lower salt levels may be required by some types of chlorinators)
Calcium Hardness Ideal level = 175 - 225 ppm
T.D.S. – Total Dissolved Solids Ideal level = less than 1500 ppm
Phosphates Ideal level = zero but less than 0.2 ppm
Water Balance Zero, or slightly negative on the Langlier Saturation Index (-0.2)
Regular Water Testing Monthly, or weekly during the swimming season

(b) Safe Handling and Storage of Pool Chemicals

  • Always read and follow all safety directions on chemical containers
  • Ensure all chemicals are stored safely and securely
  • Wear appropriate personal protective equipment when handling chemicals
  • NEVER allow chemicals to mix. Dangerous, highly corrosive fumes will be produced
  • Always add chemicals to fresh water, NEVER water to chemicals
  • Regular additions of small quantities of chemicals, as required, will result in far better results and will actually reduce the amount of chemicals used over the longer term

(c) pH – Ideal range = 7.2 – 7.6 ppm (parts per million)

pH chart

Rain may affect your pools pH depending on whether it is slightly acidic or slightly alkaline.

Ideal pH

  • 7.4 is ideal — this is the pH most comfortable to your eyes

Low pH causes problems including;

  • Aggressive water can damage mechanical components of pool
  • Irritation to eyes

High pH causes problems including;

  • Poor chlorine disinfection
  • Skin irritation
  • Calcium deposits
  • Cloudiness
  • Slippery surfaces

The primary cause of pH rise in your pool is due to the addition of chlorine which has a high pH.

Chlorine effectiveness and pH

Chlorine works most effectively at pH 7.2 to 7.6.

Note: It is very important to maintain the correct pH level by adding small quantities of acid regularly. An automated controller takes care of these regular acid additions for you.

(d) Total Alkalinity (T.A.) – Ideal range = 100 – 150 ppm (parts per million)

  • Alkalinity is the buffering system which aids in controlling the pH of your pool water
  • A correct T.A. level will result in a stable pH level, thus maintaining a stable ORP (Oxidation Reduction Potential – refer here) and stable free-chlorine levels
  • T.A. will gradually decrease over time with addition of chlorine / acid (rain can also lower your pools T.A. as it can be slightly acidic)
  • Regular addition of small quantities of alkalinity increaser (buffer) will help maintain your T.A. at its optimum level
  • A low T.A. level will allow pH to fluctuate, resulting in a corresponding variation in free chlorine level
  • A high T.A. level will require the use of more buffer and result in an increased acid and buffer demand for your pool
  • Maintaining a correct T.A. is one of the most important factors for correct operation of an automated controller

(e) Chlorine – Ideal range = 1.0 – 2.0 ppm (parts per million) but up to 3.0ppm is acceptable

Chlorine is the chemical most often used to keep swimming pools and spas free of bacteria that can be hazardous to humans.

  • Chlorine kills bacteria though a fairly simple chemical reaction
  • When chlorine is added to pool water it reacts with the water to form Hydrochloric Acid (HCl) and Hypochlorous Acid (HOCl)
  • The HOCl is the active part of the “free chlorine” that is measured on a test kit and is what sanitises the pool water
  • If the pH is too high, not enough HOCl is present and sanitisation can take much longer than normal

Chlorine comes in many forms:

  • Granular chlorine
  • Liquid chlorine
  • Tablets (stabilised)
  • Salt Water Chlorinators produce chlorine through electrolysis in a weak salt water solution (5000 ppm or less)

An automated controller will regularly add either liquid chlorine, or produce chlorine via a Salt Water Chlorinator.

Information for automated control systems using pH and ORP (Oxidisation Reduction Potential) probes

  • ORP levels to achieve 1.0 to 2.0 p.p.m chlorine vary for each pool
  • ORP is not a measure of the chlorine in pool water
  • ORP is a measure of how well the water will act to oxidise contaminants including pathogenic organisms
  • When dealing with pH/ORP controllers the water balance as a whole must always be taken into account
  • If the pH varies so will the ORP, and the free chlorine residual as well
  • Stabiliser will also affect the ORP for a given chlorine residual (i.e. low stabiliser levels will result in low chlorine residuals for a given ORP)
  • We recommend that you do not add chemicals directly to your skimmer box

Supplementary systems can be used to reduce chlorine demand and usage, and provide enhanced sanitation

  1. Ozone – Use of Ozone reduces the demand for pool chemicals by up to 60% and allows the chlorine in the pool to work more effectively as a disinfectant. Ozone actually breaks down non-living waste products in pool water.
  2. UV (Ultra-Violet) – UV systems provide an additional layer of protection beyond the capability of chlorine or salt based chlorine generators. UV reduces the amount of chlorine needed to maintain an adequate level of residual sanitiser.
  3. Ozone and UV – The combination of these two systems creates an advanced process that minimises exposure to potentially harmful chloramines and other contaminates. It dramatically reduces chemical usage by up to 80%.

(f) Stabiliser (cyanuric acid) – Ideal level = 50 ppm (parts per million)

  • Stabiliser is used to slow the destruction of free chlorine by sunlight. A pool without stabiliser can lose up to 90% of its free chlorine in just a few hours.
  • Pools without a stabiliser residual will have trouble maintaining a chlorine residual during a sunny day.
  • Stabiliser reduces the chlorine demand of the pool and so cuts the cost of pool ownership.
  • We recommend that you do not add chemicals directly to your skimmer box.
  • Liquid stabiliser is now available. No messy mixing, dissolves instantly for immediate protection.

(g) Salt (for salt chlorinated pools) – Ideal level = 3000 – 4000 ppm (lower salt levels may be required by some brands of chlorinators)

Salt water chlorination is a process that uses dissolved salt as a store for the chlorination system. The chlorine generator (or salt cell) uses electrolysis in the presence of dissolved salt (NaCl) to produce chlorine for your pool. As such, a saltwater pool is not actually chlorine-free, it simply utilizes a chlorine generator instead of direct addition of chlorine.

(h) Calcium Hardness – Ideal level = 175 – 225 ppm

Calcium hardness diagram

  • The term hardness originates from the observation that it was hard to produce foam from soap, in water with high levels of calcium and magnesium. Calcium occurs naturally in water and the amount of calcium will vary from place to place.
  • Hardness is important to understanding scale formation on pool surfaces.
  • The higher the calcium concentration in the water, the more prone the calcium is to leeching.
  • High pH levels will cause the calcium to leech out of the water and cling to the pool shell surface, as well as causing scale formation in chlorinator cells and pool heaters.

(i) Total Dissolved Solids (TDS) – Ideal level = less than 1500 ppm

Total Dissolved Solids (TDS) are the total amount of particles, including minerals, salts or metals dissolved in a given volume of water, expressed in milligrams per unit volume of water (mg/L), also referred to as parts per million (ppm). TDS is directly related to the purity of water and the quality of water purification.

Dissolved solids can come from:

Organic sources

  • Leaves and debris
  • Fertiliser and pesticides
  • Groundwater runoff

Inorganic sources

  • Materials that may contain calcium bicarbonate, nitrogen, iron phosphorous, sulphur, and other minerals.
  • Water may also pick up metals such as lead or copper as they travel through pipes used to distribute water.

Note: High TDS can result in corrosion of metal equipment and accessories, even though the water is balanced. It can cause eye and skin irritation, even though the pH is right and there are no chloramines in the water. High TDS can also permit an algae bloom, even with a 2-3 ppm chlorine residual.

(j) Phosphates – Ideal level = 0 but less than 0.2 ppm

  • Phosphate is a vital plant nutrient and the presence in swimming pool water, even at low concentrations, can cause accelerated algae growth.
  • Pools, that are properly maintained, usually do not have unexpected difficulty controlling algae, even in the presence of phosphates.
  • Higher levels of phosphates can make algae control more difficult and increase the amount of sanitizer required to maintain satisfactory control of algae.
  • While phosphate will accumulate in a pool naturally, there are steps pool owner’s can take to prevent excessively high levels.
  • Do not allow runoff from gardens and lawns to enter the pool.
  • Remove leaves from the pool regularly and promptly.
  • Have the pools water tested for phosphate levels regularly by a local pool shop.

(k) Water Balance

Water is a solvent and all things will eventually dissolve in water until the water becomes saturated. Eventually, water will become oversaturated, and excess products will be lost by precipitation. Well balanced water will increase bather comfort and will maximise the life expectancy of the pool and its components.

  • If you have a negative water balance your water is considered corrosive.
  • If you have a positive water balance your water will be scale forming, meaning that calcium can be deposited on the pool surface.

Your local pool shop can determine if your pool water is balanced or not.