How to Raise Alkalinity in a Hot Tub: Step-by-Step Guide + Expert Tips

If your hot tub alkalinity is low, you’re not alone. Many spa owners face cloudy water, pH swings, and corroding parts—all signs of imbalance. Knowing how to raise alkalinity in a hot tub is one of the most crucial skills for maintaining a clean, safe, and comfortable spa.

In this complete guide, you’ll learn why alkalinity matters , how to test and adjust it correctly , and exactly how much baking soda or alkalinity increaser to add . We’ll also cover common mistakes , troubleshooting tips , and maintenance routines used by spa professionals to keep your water stable year-round.

By the end, you’ll know how to balance alkalinity like a pro—without guesswork, over-correction, or expensive chemical mishaps.

What Causes Low Alkalinity in a Hot Tub?

Low alkalinity means your hot tub water has lost its ability to buffer pH changes . When total alkalinity (TA) drops below the recommended range of 80–120 ppm , your pH becomes unstable, drifting up and down with every new chemical, splash of water, or dose of sanitizer.

Because alkalinity acts as your water’s “shock absorber,” once it’s depleted, even a small change—like adding chlorine or rainwater—can throw off the entire balance.

Main Causes of Low Alkalinity

1. Overuse of Acidic Products: pH decreasers, sanitizers, or shock treatments with acidic bases (like sodium bisulfate or dichlor chlorine) gradually reduce alkalinity. If you frequently use these products, your buffer system weakens.
2. Source Water with Naturally Low Carbonates: Some municipal or well water sources start with low bicarbonate levels. Filling or topping off your spa with such water dilutes the existing alkalinity.
3. Excessive Rain or Snow Melt: Rainwater is slightly acidic (around pH 5.5 – 6.5). If your spa is uncovered during a storm, rainwater can significantly lower total alkalinity.
4. High Bather Load & Organic Contamination: Sweat, oils, lotions, and body care products introduce acids that eat away at the buffer capacity over time.
5. Using Chlorine or Bromine Tablets Continuously: Tablet sanitizers slowly add acidic compounds that lower alkalinity—especially in smaller spas where the water volume is limited.

Symptoms of Low Alkalinity

If your hot tub alkalinity is low , you’ll likely notice one or more of the following:

• Fluctuating pH (pH bounce): water becomes unstable, swinging from acidic to basic rapidly.
• Cloudy or green water: poor sanitizer performance and particle precipitation.
• Corrosion and staining: acidic water attacks metal fittings, heaters, and jets.
• Skin and eye irritation: acidic conditions dry out skin and make water uncomfortable.
• Reduced heater efficiency: corrosion or scale buildup on heating elements.

Why Fixing Alkalinity First Matters

Total alkalinity directly affects every other aspect of water chemistry. If you try to adjust pH before correcting alkalinity, the results won’t hold.
Professionals always balance alkalinity first , because it stabilizes pH and ensures other chemicals work effectively.

Maintaining the proper alkalinity range not only prevents corrosion but also saves money on sanitizers, reduces maintenance, and keeps water crystal clear.

Why It’s Important to Maintain Proper Alkalinity

Think of total alkalinity (TA) as the shock absorber for your hot-tub water. It neutralizes small amounts of acid and base, keeping your pH steady even when you add new chemicals, top off the tub, or soak for hours.
Without this buffering capacity, every adjustment becomes unpredictable—and water chemistry spirals out of control.

When your alkalinity is within the ideal range (80–120 ppm) , several good things happen:

• Stable pH levels: The water resists sudden changes, so you’re not constantly chasing balance.
• Better sanitizer performance: Chlorine and bromine work at peak efficiency within stable pH conditions.
• Comfortable soaks: Proper alkalinity prevents itchy skin, eye irritation, and the “chemical smell.”
• Equipment protection: Balanced water prevents corrosion of heaters, jets, and pumps.
• Cleaner surfaces: Fewer deposits or scale on acrylic hot tubs and plumbing.

Is Low Alkalinity in a Hot Tub Dangerous?

Yes — low alkalinity can seriously damage both your spa and your health over time.
Here’s why:

So when you see test results showing total alkalinity below 80 ppm , act quickly. The longer it stays low, the more stress you place on your spa surfaces, seals, and plumbing.

Why Adjust Alkalinity Before pH

A common mistake among hot-tub owners is fixing pH first. Because alkalinity buffers pH, you’ll waste chemicals if you adjust in the wrong order.

Always follow this sequence:

1. Balance Total Alkalinity → stabilize your pH buffer.
2. Then fine-tune pH → minor tweaks will now hold steady.
3. Finally, adjust sanitizer levels → chlorine or bromine will work effectively.

This “alkalinity-first” approach ensures a stable chemical ecosystem and prevents pH from bouncing back out of range after every treatment.

Read Also : Benefits of Hot tubs

Step-by-Step: How to Raise Alkalinity in a Hot Tub

When your hot tub alkalinity is low , the goal is to bring it back to the ideal 80–120 ppm range safely and steadily.
Here’s exactly how to raise alkalinity in a hot tub —whether you’re using baking soda or a commercial alkalinity increaser .

1. Test and Record Your Current Levels

Before adding anything, you need to know where your water stands.
Use spa test strips , a liquid testing kit , or a digital meter to check both total alkalinity and pH .

• Ideal Total Alkalinity: 80–120 ppm
• Ideal pH Range: 7.4–7.6

Record your readings. If alkalinity is below 80 ppm, you’ll need to increase it.

Tip: Take your sample from the center and mid-depth of the tub with jets off for 1–2 minutes to avoid inaccurate readings from surface agitation.

2. Calculate Your Hot Tub Volume

Knowing your spa’s volume ensures accurate dosing.
If you’re unsure, use this formula:

Length (ft) × Width (ft) × Average Depth (ft) × 7.5 = Volume (in gallons)

Convert to liters if needed:
1 gallon = 3.785 liters

3. Choose Your Chemical: Baking Soda or Alkalinity Increaser

Both work— sodium bicarbonate is the active ingredient in commercial alkalinity increasers.

Tip: Avoid using soda ash (sodium carbonate) unless your pH is also low , as it will raise both alkalinity and pH sharply.

4. Measure the Correct Dosage

Here’s the standard formula most spa professionals use:

1 tablespoon (≈17 grams) of baking soda per 100 gallons (≈379 L) of water raises alkalinity by about 10 ppm.

Below is a quick reference chart you can use for typical hot tub volumes:

Hot Tub Alkalinity Dosage Chart

Example:
If your 400-gallon spa tests at 60 ppm and you want to reach 100 ppm, you need to raise by 40 ppm .
Use (4 × 4 tbsp) = 16 tbsp (≈272 g) of baking soda total—add in two rounds rather than all at once.

5. Turn Off Jets and Add the Chemical

1. Switch off air valves and jets. This prevents surface agitation that can interfere with even mixing.
2. Pre-dissolve the baking soda in a bucket of spa water if possible—it prevents granules from sitting on the surface.
3. Slowly pour or broadcast the dissolved solution around the tub’s perimeter.
4. Turn jets on low for 10–15 minutes to circulate evenly.

Avoid adding near metal fittings or heaters directly. Spread the chemical evenly around the spa.

6. Wait and Retest

Let the water circulate for 3–6 hours , or ideally overnight, before retesting total alkalinity.
If the level hasn’t reached the target, add another small dose following the same process.
Gradual adjustments are safer and more accurate than adding everything at once.

7. Adjust pH If Necessary

Raising alkalinity will slightly increase pH (usually by 0.1–0.3 points).
If your pH drifts above 7.8, use a pH decreaser (sodium bisulfate) in small increments.

Remember:

Always balance alkalinity first, then fine-tune pH.

8. Retest After 24 Hours

Check both alkalinity and pH again the next day.
Your water should now be stable, clear, and within the following ranges:

• Alkalinity: 80–120 ppm
• pH: 7.4–7.6

If results drift slightly, repeat the small correction process.

Using Baking Soda vs. Commercial Alkalinity Increaser

Using Baking Soda to Raise Alkalinity

• Add 1 tablespoon per 100 gallons (10 ppm increase) .
• Safe, affordable, and effective for DIY adjustments.
• Great for mild to moderate corrections.

Using a Spa Alkalinity Increaser

• Follow product label—usually 2 oz (56 g) per 500 gallons raises TA by ~10 ppm .
• Dissolves faster and ensures consistent pH balance.
• Recommended for large spas or commercial use.

Pro Tip: Keep a small kitchen scale near your spa station—precision prevents overshooting and chemical waste.

When to Use a Professional Product

If you’re managing persistent low alkalinity or a large commercial spa , commercial products with stabilizers may be better.
Look for ingredients like sodium bicarbonate and borate buffers that prevent rapid pH drift.

Troubleshooting & Edge Cases When Adjusting Alkalinity

Even when you follow the correct steps, water chemistry can be unpredictable. Understanding why your alkalinity won’t stabilize , or how to fix overcorrection , is key to maintaining a consistently healthy hot tub .

1. Overcorrecting Alkalinity (Too High After Adjustment)

If your alkalinity is above 120–150 ppm , the water becomes “over-buffered.” This leads to a condition known as pH lock , where your pH refuses to change despite adding acid.

How to Lower Alkalinity in a Hot Tub

You can reduce alkalinity safely using one of these two methods:

Option 1 – pH Decreaser (Dry Acid / Sodium Bisulfate):

1. Turn off all jets and air valves.
2. Pre-dissolve 1 oz (28 g) of sodium bisulfate per 500 gallons (1,900 L) to reduce alkalinity by around 10 ppm.
3. Pour the solution into the deepest area of the tub, away from metal fittings.
4. Leave the water still for 2–3 hours , then turn jets on low to circulate.
5. Retest after 6–8 hours, and repeat if necessary.

Option 2 – Muriatic Acid (Advanced Users Only):

• Handle with gloves and goggles .
• Use outdoors or in a well-ventilated area.
• Add very small doses (¼ cup per 400 gallons), then retest.

Pro Tip: Avoid making large corrections all at once. Reducing alkalinity too quickly can cause a steep pH drop that irritates skin and damages spa finishes.

2. “pH Lock” – When pH Won’t Move

High alkalinity “locks” pH in place. Adding pH decreaser barely changes readings because the water’s buffering capacity resists change.

To fix pH lock:

• Lower alkalinity first with acid (as above).
• Once alkalinity drops below 120 ppm, you’ll regain control over pH.
• Then rebalance your sanitizer levels.

3. Alkalinity Keeps Dropping Repeatedly

If you find yourself constantly adding baking soda, there may be an underlying issue:

4. Can You Raise Alkalinity Without Increasing pH?

This is one of the most frequently asked questions—and the short answer is: not entirely.

Because alkalinity and pH are chemically linked , increasing total alkalinity will slightly raise pH. However, there are techniques to minimize that rise:

• Add the chemical in small increments , retesting every few hours.
• Circulate gently (low jet speed) instead of vigorous aeration, since air exposure boosts pH.
• Consider borate buffers (30–50 ppm) — they stabilize pH without affecting alkalinity.

You can’t completely separate pH and alkalinity, but you can control how dramatically pH changes when raising alkalinity.

5. Old or Cloudy Water (High TDS)

When water has been in the tub for months, minerals, oils, and contaminants accumulate. This increases total dissolved solids (TDS) , making alkalinity adjustments ineffective.

If your alkalinity never stabilizes or the water stays cloudy despite corrections:

• Drain, clean, and refill the hot tub.

Start fresh with properly balanced water—it’s faster and safer than endlessly tweaking chemistry.

6. Local Water Source & Hardness Effects

Your tap water chemistry plays a major role in alkalinity behavior:

7. When to Drain and Start Fresh

Replace your spa water if:

• Alkalinity or pH fluctuates daily despite adjustments.
• Water feels slimy, foamy, or smells “off.”
• It’s been more than 3–4 months since your last change.
• You’ve had multiple overcorrections or chemical build-ups.

A complete drain and refill resets your chemistry to a clean baseline, saving time and frustration.

Read Also  : Benefits of Hot tubs