How to lower alkalinity in a hot tub
pH down, also called sodium bisulfate or dry acid, is the short answer for how to lower alkalinity in hot tub water. Apply it in small batches until your strip shows 80 to 120 ppm.
Choose your tub and enter your alkalinity reading, and the dose appears here.
- Range for total alkalinity
- 80 to 120 ppm
- Amount of pH down per 10 ppm for every 100 gallons
- 0.34 oz (10 g)
- The largest amount of dry acid for a single round, per 500 gallons
- 1 oz (28 g)
- Target level for the dose
- 100 ppm
How much does pH down lower alkalinity?
This rate comes from the CPO dosage tables on sodium bisulfate. For each 10 ppm, the tables call for 0.21 lb, or 95 g, per 1,000 gallons, which is 3,785 litres. Each dose moves the reading to about 100 ppm.
| Tub size | 140 ppm | 160 ppm | 180 ppm | 200 ppm | 240 ppm |
|---|---|---|---|---|---|
| 180 gal (670 L) | 2.4 oz (69 g) | 3.7 oz (104 g) | 4.9 oz (139 g) | 6.1 oz (173 g) | 8.6 oz (243 g) |
| 250 gal (930 L) | 3.4 oz (96 g) | 5.1 oz (145 g) | 6.8 oz (193 g) | 8.5 oz (241 g) | 12 oz (337 g) |
| 290 gal (1,100 L) | 3.9 oz (112 g) | 5.9 oz (168 g) | 7.9 oz (224 g) | 9.9 oz (280 g) | 14 oz (391 g) |
| 350 gal (1,330 L) | 4.8 oz (135 g) | 7.1 oz (202 g) | 9.5 oz (270 g) | 12 oz (337 g) | 16.5 oz (472 g) |
| 500 gal (1,900 L) | 6.8 oz (193 g) | 10 oz (289 g) | 13.5 oz (386 g) | 17 oz (482 g) | 24 oz (675 g) |
A 290 gallon (1,098 litre) tub needs 0.37 lb (168 g) to drop levels by 60 ppm, says the Intex manual. This figure fits the 160 ppm cell. Follow your label if it differs, since the Leslie's chart asks for more for its brand.
How to lower alkalinity in hot tub water in rounds
Small rounds work best when using acid. AquaChek's water care guide limits dry acid to 1 oz (28 g) per 500 gallons (1,900 litres) for each round.
- Check both pH and alkalinity levels. Read both pads using the test strip guide.
- Calculate the total amount needed. Get this from the chart or the chemical calculator for your size.
- Dissolve a single round of the chemical. In a plastic bucket of tub water, stir in up to the cap.
- Pour it in after turning off the pump and turning on the air blower. This prevents an acid surge from reaching the heater and pump, says AquaChek.
- Let 30 minutes pass, then test again. Add the next round after that.
- Stop once you are near 100 ppm. Check pH next, since it drops as well.
Dry acid has corrosive properties. Wear eye protection, and add the acid to the water, never the other way round. The Intex manual warns to add products separately and never mix two together.
Hot tub alkalinity too high: signs and why
Frog says cloudy water is the most common issue caused by high alkalinity. Most other signs stem from that same cause.
| Sign | Why it happens |
|---|---|
| Cloudy or milky water | Minerals leave the water and float within it |
| Scale on the shell and equipment | Those minerals settle on whatever the water touches |
| Clogged filter, weaker flow | Scale and particles block the filter and pipes |
| pH that will not come down | The water resists change, which Frog calls pH lock. |
| Chlorine that swings | Lay-Z-Spa lists it with high alkalinity |
Other factors can cause haze too. Work through cloudy hot tub water if haze stays once levels are right.
Why does pH drop too?
Both values drop with pH down. Frog recommends testing pH after every dose because alkalinity reduction can pull pH down too.
If alkalinity stays high but pH goes under 7.2, handle alkalinity first. Only after alkalinity is in range does the Intex manual say to adjust pH. Next, use soda ash in a small dose to raise the pH.
Things that raise alkalinity, and your tub's own range
Lotion, makeup, hard fill water with high pH, and old water are cited by Lay-Z-Spa. When water is old, a fresh fill works better than repeated acid doses.
The EPA-registered chlorine label uses 80 to 120 ppm, like most guides. Some manufacturers set a higher target range. A range of 125 to 150 ppm appears in the Hot Spring manual, so follow your manual if it sets one.
Common questions
If alkalinity runs too high in a hot tub, what happens?
Scale builds on the shell and equipment as the water turns cloudy. High pH also stays stuck and is difficult to lower.
Is it possible to lower alkalinity without dropping pH?
Not if you use pH down, as it lowers both. Start by lowering alkalinity, then restore pH to 7.4 to 7.6 using a small dose of pH up.
Is it OK to use muriatic acid in a hot tub?
In spas, AquaChek recommends avoiding it. Very small amounts work because it is so strong, which makes dry acid less tricky to measure.
How much time will you need to reduce alkalinity?
A large drop takes several rounds, with about 30 minutes needed before you test again for each one. After the final round, Intex says to wait 24 hours and test again.
Is draining the tub the right move to lower alkalinity?
If the water is old, draining can be useful. Among the causes, Lay-Z-Spa includes leaving water in too long. Check the new water before you put anything in it.