Pool Chemistry Glossary

Backwash

Reversing water flow through a sand or DE filter to flush out trapped debris. Backwashing removes water from the pool (which dilutes stabiliser slightly over time). Top up with fresh water after backwashing.

Cal-Hypo (calcium hypochlorite)

Granular chlorine, usually 65–70% available chlorine. More concentrated than liquid chlorine by weight. Does not raise stabiliser levels. Pre-dissolve in a bucket before adding to the pool.

Calcium hardness

The amount of dissolved calcium in your pool water. Too low and the water becomes corrosive (it draws calcium from grout, plaster, and equipment). Too high and calcium deposits form on surfaces and in pipes. Target: 200–400 ppm.

Chloramines

Combined chlorine compounds formed when chlorine reacts with ammonia from sweat, urine, and other organics. They cause the "pool smell", eye irritation, and skin irritation. Shocking breaks them down.

Combined chlorine

Chlorine that has already reacted with ammonia or organic matter and is no longer effective. High combined chlorine causes the "pool smell" that people often mistake for too much chlorine. Shocking the pool breaks it down.

Dry acid (sodium bisulphate)

A granular alternative to muriatic acid for lowering pH. Easier to handle than liquid acid but the same caution applies — dissolve in water before adding to the pool.

Free chlorine

The active chlorine in your pool that is available to kill bacteria and algae. This is the number you are trying to keep at 1–3 ppm. Distinct from "combined chlorine" (used-up chlorine that has already reacted with contaminants).

Muriatic acid

A diluted form of hydrochloric acid used to lower pH or alkalinity. More powerful than dry acid (sodium bisulphate) — effective but requires careful handling, dilution, and good ventilation.

pH

A measure of how acidic or alkaline your pool water is, on a scale from 0 to 14. The ideal range is 7.2–7.6. Below 7.2 the water is corrosive and irritates eyes and skin; above 7.6 chlorine becomes much less effective.

ppm (parts per million)

The unit used for most pool chemistry measurements. 1 ppm means 1 milligram of the substance per litre of water — an extremely small concentration. At 1 ppm free chlorine, a standard pool contains only a few grams of active chlorine in total.

Shocking

Temporarily raising the free chlorine to a high level — typically 10–40 ppm depending on your stabiliser level — to kill bacteria, algae, and break down chloramines. Not the same as regular chlorination.

SLAM (Shock, Level, And Maintain)

A method for clearing a green or cloudy pool by holding chlorine at shock level continuously until the water clears and tests pass. Developed by the Trouble Free Pool community.

Stabiliser / CYA / cyanuric acid / conditioner

All names for the same thing: a chemical that protects chlorine from being destroyed by sunlight. Without it, outdoor pools lose most of their chlorine within a few hours. Target range: 30–50 ppm. Too much stabiliser makes chlorine ineffective.

Total alkalinity (TA)

A measure of the water's ability to resist changes in pH. Think of it as a buffer — low alkalinity means pH swings wildly with small chemical additions. Target range is 80–120 ppm. Adjust alkalinity before adjusting pH.

Total chlorine

Free chlorine plus combined chlorine. Some test strips measure total chlorine rather than free — check your strip's label. Most modern dip-and-read strips measure free chlorine, but it is worth confirming. For water safety, free chlorine is the number that matters.

Trichlor tablets

A common form of slow-release chlorine for pools, usually sold as 200g tablets for floaters. Each tablet contains around 57% cyanuric acid — so regular use raises stabiliser levels over time. Test stabiliser monthly if you use tablets.