pH & Alkalinity Reducer - Sodium Bisulphate

Bring high hot tub or pool pH down safely using sodium bisulphate, better known as dry acid. It settles pH into the 7.2-7.6 band where sanitiser works best, and does it without the fumes you get from liquid acid.

• Drops pH by around 0.2 units for every 16 g added per 1000 L.
• Pulls total alkalinity down at the same time, so measure carefully.
• Brings back chlorine and bromine sanitising power, which falls away above pH 8.
• Keeps calcium scale off heaters, jets and waterline tiles.
• Removes the skin and eye irritation that alkaline water causes.

Sizes and prices

Available in 1kg, 2.5kg, 5kg, 7kg, 10kg, 14kg, 20kg, 25kg.

• 1kg: £5.89
• 2.5kg: £7.95
• 5kg: £11.38
• 7kg: £14.12
• 10kg: £18.23
• 14kg: £23.72
• 20kg: £31.94
• 25kg: £38.80

How to use

1. Test pH and total alkalinity with a fresh strip or liquid kit.
2. Work out the dose from the table below, then add no more than half of it to start with.
3. Switch the circulation pump on and take out the floating dispenser.
4. Dissolve the dose fully in a clean bucket of cool tub water, about 5 L, stirring until it turns clear.
5. With the pump running, pour the dissolved solution slowly across the surface.
6. Leave it 30-60 minutes, then retest pH and alkalinity before adding any more.

Frequently asked questions

What pH should a hot tub sit at?

7.2-7.6 for hot tubs on chlorine or bromine. Stay under 7.8 so the sanitiser keeps working.

Does pH minus lower alkalinity too?

Yes. Sodium bisulphate brings both pH and total alkalinity down at much the same rate.

When can I get in after dosing?

Give it 60 minutes with the pump running, then retest. If pH is in band you are good to bathe.

Why pick dry acid over liquid muriatic acid?

Granules store and travel more safely, give off no fumes, and are easier to dose accurately at home.

How far will 5 kg go?

Around 80-100 typical doses for a 1500 L hot tub, or a year of monthly maintenance on a 30 m3 pool.

Can I use it in a salt-water pool?

Yes. Salt chlorinators tend to run pH high, so you will reach for pH minus more often than with a tablet setup.

Why is chlorine more effective at lower pH?

Below pH 7.5 chlorine sits as hypochlorous acid (HOCl); above it as hypochlorite ion (OCl-). HOCl is around 80 times better at killing bacteria.

Does sodium bisulphate add sulphates?

Yes. Each 100 g per 1000 L adds roughly 80 ppm sulphate, so refresh some water if sulphates pass 300 ppm.

Why dose half and then retest?

Heated water reacts quicker than you might expect. Half-dosing keeps pH from crashing below 7.0 and avoids etching.

What do I do if I overdose?

Add an equal weight of pH plus (sodium carbonate) to bring it back up, then retest after 60 minutes.

Why does my pH keep creeping back up?

Either total alkalinity is over 150 ppm or your tap water is dosing carbonates in. Lower TA first to stop the drift.

Why has foam appeared since I lowered pH?

Acidified water can release dissolved organics as foam. Add Foam Away at 25 ml per 1000 L and clean the filter.

pH minus or pH plus?

pH minus (sodium bisulphate) lowers pH and TA. pH plus (sodium carbonate) raises pH with little effect on TA.

pH minus versus muriatic acid?

Same chemistry, different form. Muriatic acid is quicker but far more hazardous to handle, so sodium bisulphate is the domestic-safe alternative.