Pool water chemistry basics for home owners

Pool water chemistry can sound intimidating, with talk of ppm, pH, alkalinity and stabiliser. In practice, home pool care comes down to a few principles: keep a sanitiser working to kill germs and algae, keep the water balanced so the sanitiser works and your pool equipment lasts, and test regularly so you catch problems early.

This guide explains the basics in plain English. It is general information only. Pool types, equipment and chemicals vary, so always follow the instructions on product labels and the advice of your pool builder, equipment manufacturer or a qualified pool professional. Public pools are subject to state health guidelines; home pools generally aren't, but those guidelines can be a useful reference.

Sanitising: keeping germs and algae away

The most important job in pool care is sanitising. Most Australian home pools use chlorine, either added directly (liquid, granules or tablets) or generated from salt by a saltwater chlorinator. Chlorine kills bacteria and other microorganisms and helps prevent algae.

There are two key measurements:

• Free chlorine is the chlorine available to sanitise.
• Combined chlorine (chloramines) is chlorine that has already reacted with contaminants.

A strong "chlorine smell" is often caused by chloramines, not by too much chlorine. Shocking (a larger dose of sanitiser) is commonly used to break them down.

pH

pH measures how acidic or alkaline the water is, on a scale where 7 is neutral. Pool water is usually kept slightly alkaline. Wikipedia's swimming pool sanitation article notes that a pH of around 7.2 to 7.8 is commonly recommended:

• If pH is too high, chlorine works less effectively, and scale can form.
• If pH is too low, water can irritate eyes and skin and corrode equipment and surfaces.

pH tends to rise in many pools, especially saltwater pools, so acid is often added to bring it down. Follow the product label carefully.

Total alkalinity

Total alkalinity acts as a buffer that helps keep pH stable. If alkalinity is too low, pH can swing wildly. If it's too high, pH can be hard to adjust. Your test kit or pool professional can tell you the right range for your pool.

Calcium hardness

Calcium hardness is the amount of dissolved calcium in the water. Too little can make water aggressive towards concrete and grout; too much can cause scaling on surfaces and equipment, including salt cells. The right range depends on your pool surface, so check with your builder or pool professional.

Stabiliser

Outdoor pools lose chlorine quickly in sunlight. Stabiliser, usually cyanuric acid, helps protect chlorine from UV breakdown. But too much stabiliser can reduce chlorine's effectiveness. Because stabiliser doesn't break down easily, levels can build up over time, especially if you use stabilised chlorine products. Test it periodically.

Testing your water

You can test water with:

• test strips, which are quick and convenient;
• liquid or tablet test kits, which can be more precise;
• digital testers;
• free or paid testing at pool shops, where a sample is analysed.

Test sanitiser and pH regularly, at least weekly for many pools, and more often in hot weather, after heavy use or after rain. Test alkalinity, calcium and stabiliser less often.

Use our pool calculator to estimate your pool's volume, which you need for dosing chemicals correctly.

Common problems and causes

Circulation and filtration

Chemicals work best when water circulates and is filtered well. Run your pump long enough for the water to turn over regularly, keep skimmer baskets and pump strainers clear, and clean or backwash the filter as your filter manufacturer recommends. Brushing walls and floors helps prevent algae from getting a foothold.

Storing chemicals safely

• Keep chemicals in their original, labelled containers.
• Store them in a cool, dry, ventilated place, away from sunlight and heat.
• Keep different chemicals apart, especially chlorine products and acids.
• Lock them away from children and pets.
• Don't store chemicals inside the pool barrier where children could reach them.
• Dispose of old or unwanted chemicals through your council's household chemical collection.

After rain and heavy use

Heavy rain can dilute sanitiser and change pH. Pool parties increase contamination from sweat, sunscreen and urine. After either, test the water and adjust. A shower before swimming and regular toilet breaks for children help keep water cleaner.

Keeping a pool logbook

A simple logbook makes water care much easier. Record:

• the date and your test results for sanitiser, pH and, less often, alkalinity, calcium and stabiliser;
• chemicals added, with amounts;
• weather events such as heavy rain or heatwaves;
• filter cleaning, backwashing and equipment servicing.

Over a season, the log shows patterns, such as how quickly chlorine drops in hot weather, and helps a pool professional diagnose problems if you need help.

Common mistakes

• Adding chemicals without knowing the pool volume.
• Mixing chemicals or adding water to chemicals.
• Shocking the pool while swimmers are in the water.

Key points

Keep a sanitiser such as chlorine working to kill germs and algae, and keep water balanced so it works well. Pool pH is commonly kept around 7.2 to 7.8, with alkalinity, calcium hardness and stabiliser managed as your pool type requires. Test regularly, especially in summer, circulate and filter well, and handle chemicals safely: never mix them, store them separately and keep them away from children.

Related guides

Read saltwater versus chlorine pools, pool heating and running costs and CPR and first aid for pool owners.