Stabilised outdoors where sunlight would destroy the residual, unstabilised indoors and wherever cyanuric acid has already accumulated.

Cyanuric acid protects chlorine from ultraviolet degradation. Without it, an outdoor pool can lose most of its free chlorine in a single sunny day, which makes maintaining a residual impossible.

The problem is that cyanuric acid is not consumed. Every stabilised dose adds more, and the only way it leaves is by dilution or backwashing.

At elevated levels it binds chlorine more tightly than is useful, so the measured free chlorine becomes progressively less effective. Most standards therefore set a maximum, and the remedy is partial draining and refilling.

Indoor pools have no ultraviolet exposure and need no stabiliser at all, so unstabilised products are the correct routine choice there.

Test cyanuric acid periodically. A pool that suddenly needs much more chlorine for the same result is often a stabiliser problem rather than a demand problem.

Tablets for steady maintenance through a feeder; granules for rapid correction and for pools where a controlled slow release is not wanted.

Tablets are compressed and dissolve slowly. Placed in a floating dispenser, a skimmer basket or an inline feeder, they release chlorine continuously between service visits, which is what keeps a residual stable.

Granules dissolve quickly and are used when the level needs raising now - after heavy bather load, after rain, or when a test comes back low.

Feeders give better control than skimmer baskets. A tablet in a skimmer produces a concentrated acidic stream through the pump and heater whenever the pump runs, and corrodes equipment over time.

Granules should be pre-dissolved in a bucket of water and added to the pool rather than broadcast, particularly on vinyl and painted surfaces where undissolved granules bleach and damage the finish.

Most operations use both: tablets for baseline, granules for correction.

Whatever the local standard requires - and it differs between jurisdictions and between domestic and public pools.

Public and commercial pools are usually subject to a national or regional standard specifying a free chlorine range, a maximum combined chlorine, a pH range and testing frequency, with records to be kept.

Domestic pools are generally unregulated but the same chemistry applies, and manufacturer guidance follows similar ranges.

The residual required rises with bather load, temperature and organic input, so a single figure is a starting point rather than an answer. Spa pools typically need considerably more than a swimming pool.

pH governs how effective that residual is, which is why the two are always managed together.

Establish the applicable standard for the pool's classification and location, and set the operating target inside its range rather than at its edge.

Because mixing chlorine types - or contaminating one with another - can cause a violent reaction or fire, and it has done so repeatedly.

Trichlor and calcium hypochlorite are incompatible. Mixing them, or putting one into a feeder that still holds residue of the other, can generate enough heat to ignite.

This is not theoretical - it is a recurring cause of fires at pool plant rooms and domestic stores.

The rules are simple and absolute: one product per feeder, never combine products in a container, use a clean dry scoop for each, and never return unused product to the wrong tub.

Storage segregation matters equally. Keep chlorine products apart from acids, from each other, and from any organic material, in a cool dry ventilated place.

Where a feeder is being changed from one product to another, it must be emptied and thoroughly rinsed first.

Trichlor and dichlor lower pH and add stabiliser; calcium hypochlorite raises pH and adds calcium; sodium hypochlorite raises pH strongly.

Trichlor tablets are acidic. Continuous use drives pH and total alkalinity down, which eventually requires alkalinity increaser and pH correction.

Dichlor granules are closer to neutral but still add cyanuric acid with every dose.

Calcium hypochlorite raises pH and adds calcium hardness. In hard-water areas that accelerates scaling on surfaces, heat exchangers and filters.

Sodium hypochlorite raises pH noticeably and adds no stabiliser or calcium, which makes it common on commercial installations with automated pH correction.

The practical consequence is that the sanitiser choice determines what else has to be dosed. A pool on trichlor will consume alkalinity increaser; a pool on calcium hypochlorite will consume acid.

Cool, dry, ventilated, in original closed containers, segregated from acids and from other chlorine types, and away from anything combustible.

Chlorine donors are oxidisers. Contact with organic material, oils, or incompatible chemicals can start a fire, and moisture accelerates decomposition.

Containers must be kept closed. An open tub takes up atmospheric moisture, degrades and can release gas into the store.

Store below eye level and never above head height, because a leaking container of chlorine product above the face is a serious hazard.

Ventilation matters. Plant rooms and stores holding pool chemicals should be ventilated to the outside, and acid should be stored in a separate area entirely.

Keep the safety data sheets accessible, and make sure whoever handles the products knows which combinations are prohibited.

Free chlorine is the active sanitiser, combined chlorine is spent chlorine bound to contaminants, and total is the sum of the two.

Free chlorine is what disinfects. It is the number that matters for whether the water is safe.

Combined chlorine - chloramines - forms when chlorine reacts with nitrogen compounds from bathers. It has very little sanitising power and is responsible for the sharp smell and eye irritation wrongly attributed to 'too much chlorine'.

A pool that smells strongly of chlorine usually has too much combined and not enough free.

Most standards cap combined chlorine at a low proportion of free, and exceeding it indicates the pool needs shock treatment and probably more fresh water.

Testing must distinguish them. A total chlorine reading alone can look adequate while the free chlorine that actually does the work is nearly absent.