Hot Tub Water Chemistry: A Beginner's Guide

Good water chemistry keeps your hot tub safe, comfortable, and protects the equipment from corrosion and scale. It sounds technical but once you understand what each parameter does and how to adjust it, it becomes routine.

pH — the most important number

pH measures how acidic or alkaline your water is on a scale of 0–14. For hot tubs, you want it between 7.2 and 7.6.

• Too low (acidic): Itchy eyes and skin, corrosion of metal components, degraded seals and jets
• Too high (alkaline): Cloudy water, scale build-up on the heater element, reduced sanitiser effectiveness

To raise pH: add pH Plus (sodium carbonate). To lower it: add pH Minus (sodium bisulphate). Always add chemicals to water, never the other way around.

Total Alkalinity (TA)

Alkalinity is the buffer that stabilises your pH. Get this right first, and pH becomes much easier to control. Target 80–120 ppm.

• Low TA causes pH to swing wildly up and down — you'll constantly be chasing it
• High TA makes pH stubbornly resistant to change and causes cloudy water

To raise alkalinity, use Alkalinity Increaser (sodium bicarbonate). To lower it, use pH Minus added in smaller doses with the jets running.

Sanitiser — chlorine or bromine?

Your sanitiser kills bacteria and keeps the water safe. The two most common options are:

• Chlorine — faster acting, cheaper, and the most common choice. Target 3–5 ppm. Granular chlorine is easiest for hot tubs.
• Bromine — gentler on skin and eyes, more stable at high temperatures, better for people sensitive to chlorine. Target 3–5 ppm.

Shock treatment

Shocking oxidises contaminants that build up in the water — body oils, dead organic matter, and combined chlorine (chloramine) that causes the "chlorine smell". Shock your tub:

• Weekly, as routine maintenance
• After heavy use (parties, multiple bathers)
• After periods of non-use
• When the water looks dull or smells off

Use a non-chlorine shock (potassium monopersulphate) weekly — it won't spike your chlorine levels and you can bathe sooner after dosing.

Calcium Hardness

This measures the dissolved calcium in your water. Target 150–250 ppm. Lincolnshire has moderately hard water, so you may not need to add much.

• Too low: Soft water is aggressive and will corrode acrylic, metal and rubber components
• Too high: Scale forms on the heater element, reducing efficiency and lifespan

How to test your water

Use test strips weekly and a liquid test kit monthly for more accurate readings. Always test before adjusting — never dose blind.

The correct order for adjusting chemicals is:

1. Total Alkalinity first
2. pH second
3. Calcium Hardness
4. Sanitiser last

Wait 30 minutes between adjustments with the jets running, then re-test before adding anything else.

When to drain and refill

Even with perfect chemistry, total dissolved solids (TDS) build up over time. As a rule, drain and refill your hot tub every 3 months, or whenever chemicals stop responding as expected.

All our maintenance packages include water testing and balancing at every visit. If you'd like us to take this off your hands, get in touch.