Does algae cause high pH in pool?

Algae growth can contribute to high pH levels in swimming pools for a few key reasons. In this article, we’ll explore the connection between algae and pH in pool water and how to prevent and treat high pH caused by algae.

Quick Answers

– Algae photosynthesis removes carbon dioxide from water, raising pH over time.
– Dead algae decompose, producing ammonia that raises pH.
– Some algaecides contain chemicals like copper that raise pH.
– Regular algae treatment, scrubbing, and water balance help prevent pH spikes.
– Baking soda or muriatic acid lower high pH from algae back to ideal range.

How Does Algae Raise pH in Pools?

Algae can raise pH in swimming pools through two key mechanisms:

Photosynthesis Removes Carbon Dioxide

Living algae perform photosynthesis, using energy from the sun to convert carbon dioxide (CO2) and water into glucose and oxygen. This process removes carbon dioxide, which acts as a weak acid, from the pool water. With less CO2 present, the water loses some acidity and the pH rises.

The more algae grows and spreads in a pool, the faster it consumes CO2 through photosynthesis during the day. In pools with significant algal growth, the pH can climb rapidly and reach uncomfortably high levels above 8.0.

Decomposing Algae Release Ammonia

As algae accumulate, die, and decompose in a swimming pool, they release ammonia into the water as a byproduct. Ammonia acts as a base and directly increases the pH.

Even after treating and removing living algal growth, dead algal material can linger in the water or attached to pool surfaces. As this dead algae break down, they release surges of ammonia that drive up pH.

In poorly maintained pools, accumulated algal debris can cause pH to spike above ideal ranges and make the water unsafe.

Do Algaecides Increase pH?

Some algaecide products intended to kill and prevent algal growth contain ingredients like copper or silver compounds that act as algaestats.

These metals can inhibit algae reproduction and metabolism. However, they also tend to increase pH in pool water after application.

Other quaternary ammonium algaecides may not directly affect pH. But by killing algae, they can release organic matter that decomposes into ammonia and raises pH.

Using an algaecide is not an excuse to avoid manually removing algae and proper pool maintenance. Products that include copper or silver typically require occasional pH adjustment after treatment.

Preventing High pH From Algae Growth

To help prevent algae-related spikes in swimming pool pH:

– Maintain proper chlorine sanitizer levels between 1-3 ppm.
– Shock pool water periodically with chlorine or potassium peroxymonosulfate (Vanish, Oxy-Brite).
– Use an EPA registered algaecide according to label directions when needed.
– Remove visible algae by scrubbing walls, floors, and equipment.
– Vacuum and backwash filters to remove debris.
– Maintain total alkalinity around 80-120 ppm.
– Keep phosphate levels below 100 ppb (0.1 ppm).
– Provide proper pool circulation and filtration for turnover rate.
– Adjust cyanuric acid to 30-50 ppm in outdoor pools.

Maintaining proper water balance also helps starve algae growth by keeping pH, alkalinity, calcium, and minerals at ideal ranges.

Testing water chemistry at least weekly helps catch pH and sanitizer changes before algae takes over. Addressing issues early prevents more severe algae blooms.

Treating High pH Caused by Algae

If algae already caused pH to climb too high, bringing it back down requires removing algae, debris, and dead organic matter first through scrubbing, vacuuming, filtrating, and an algaecide treatment.

Once algae and waste are reduced, pH can be lowered by:

– Adding muriatic acid or sodium bisulfate to lower total alkalinity, which will directly reduce pH.
– Using a pH and alkalinity reducer product.
– Diluting the pool water with fresh water if pH is extremely high. This may take several rounds to slowly bring pH back down.
– Running the pool pump and filter 24 hours a day until pH is back in range. This removes algal debris faster.

Muriatic acid is the most reliable and quickest way to lower pH and total alkalinity after an algae bloom, but it must be handled with care. Broadcasting powdered sodium bisulfate is safer and easier for do-it-yourself pool owners.

Add acidifier slowly and retest pH every few hours until the desired level is reached. The goal is to lower pH to ideal range around 7.4-7.6 without overshooting. Bringing it down too far makes chlorine less effective.

Once the crisis is under control, rebalancing total alkalinity back up to 80-120 ppm using baking soda will help buffer against future pH swings.


Uncontrolled algae growth can lead to high pH in swimming pools through photosynthesis and decomposition. While algaecides may contribute to pH increase, they are not a substitute for manual cleaning and water balance maintenance to control algae in the first place. Bringing pH back down after an algal bloom requires removal of algae and debris, use of muriatic acid or bisulfate, dilution, and preventing future blooms with better chlorine and algae control.

Frequently Asked Questions

What is the ideal pH range for pool water?

The recommended pH range for a swimming pool is 7.4 to 7.6. This level provides maximum sanitizer efficiency and helps prevent corrosion or scaling. Allowing pH to rise above 7.8 makes chlorine less effective against algae and microbes.

Should I use baking soda if my pool pH is too low?

Yes, adding baking soda is the best way to raise pH if it drops below the ideal range. Baking soda increases total alkalinity, which buffers against large pH changes. Add in small amounts and test every few hours until desired pH is reached.

Why does cyanuric acid raise the pH in my pool?

Cyanuric acid stabilizes chlorine in outdoor pools from UV rays. However, it can slowly hydrolyze and release cyanurate ions that increase pH. Keep cyanuric acid levels 30-50 ppm and monitor pH more frequently when using it.

How often should I test my pool’s pH level?

Test and adjust the pH at least 2-3 times per week in summer when the pool is being used frequently. Also test pH after adding chemicals, after heavy rain or long periods of non-use, and any time the water appears cloudy, green, or algae-covered.

What’s the quickest way to lower high pH caused by algae?

The fastest method is using muriatic acid (hydrochloric acid) to lower total alkalinity and pH together. Broadcast or carefully pour acid around edges of the pool while running the pump. Retest and add in increments every few hours until target pH is reached.

Comparing Pool Chemicals for pH Control

Chemical Effect on pH Considerations
Muriatic Acid Lowers pH Fast acting but handle with care. UsePersonal protective equipment recommended.
Baking soda Raises pH Slow acting buffer. Safer for DIY use. Avoid overshooting pH.
Borax Raises pH slightly Safer substitute for baking soda. Mix with soda ash for more impact.
Sodium bisulfate Lowers pH Granular acidifier safer than muriatic for residential use.

Using Baking Soda to Raise Low Pool pH

What is baking soda?

Baking soda, or sodium bicarbonate (NaHCO3), is a mild base used to increase pH and total alkalinity in pools. It dissociates into sodium (Na+) and bicarbonate (HCO3-) ions in water.

How does baking soda increase pH?

When dissolved, the bicarbonate ions react with hydrogen ions (H+) and remove them from solution. This reduces water’s acidity, thereby increasing the pH. Total alkalinity rises since bicarbonate is an alkalinity buffer.

How much baking soda should be added?

For simple pH increase, add 2.0-4.0 lbs of baking soda per 10,000 gallons of pool water to increase pH around 0.1-0.2 points. Pre-dissolve before adding to pool. For low pH emergency, up to 6 lbs per 10,000 gallons can be used with caution.

Is there an easier way to add and dissolve baking soda?

Some methods to dissolve and add baking soda easier include:

– Mixing baking soda with water in a bucket before pouring into pool
– Broadcasting powder into turbulent areas like pool inlets and waterfalls
– Using a feeder or erosion-style chlorinator to s lowly dissolve soda
– Adding through pool cleaner suction for improved mixing and dispersion

How long until pH rises after adding baking soda?

Measure pH change about 2-4 hours after adding dissolved baking soda. The full effect may take up to 24 hours. Test pH incrementally and add more baking soda if target pH is still not reached.

Can too much baking soda be added?

Yes, adding excessive amounts can push pH and alkalinity too high. This wastes money and chemicals. Go slowly and test repeatedly until target pH is achieved. Generally don’t exceed 6 lbs per 10,000 gallons at a time.

Lowering Pool pH with Muriatic Acid

What is muriatic acid?

Muriatic acid is an industrial name for hydrochloric acid or hydrogen chloride (HCl) in solution. It is a strong mineral acid used to lower pH and alkalinity in pools.

What safety gear is needed when using muriatic acid?

Always wear eye protection, rubber gloves, and closed toe shoes when handling muriatic acid. Work in a well-ventilated outdoor area. Keep ample water nearby in case of skin contact.

How does muriatic acid reduce high pH?

When added to pool water, hydrogen ions (H+) from hydrochloric acid react with and neutralize excess hydroxide ions (OH-), lowering pH. Hydrochloric acid also reduces total alkalinity.

How much muriatic acid should be added to lower pH?

Start by adding 1 quart of acid per 10,000 gallons of pool water. Broadcast around the perimeter with the pump running. Retest pH after 2-4 hours and add more in 1 quart increments as needed.

Is there a less harsh option than muriatic acid?

Dry acid products like sodium bisulfate are slower acting but safer options for residential pool pH reduction. However, muriatic acid remains the quickest and most effective acidifier option for severe high pH situations caused by algae.

Can I put muriatic acid directly in the pool or skimmer?

No. Always add muriatic acid slowly around the pool edges. Pre-diluting it first with a bucket of pool water also helps avoid localized damage or etching. Never add any pure chemicals directly into the skimmer.

Preventing Algae Growth in Swimming Pools

Algae prevention is a crucial part of pool maintenance. Here are top tips for keeping algae at bay:

Maintain Proper Chlorine Levels

– Check chlorine level at least 2-3 times weekly.
– Keep free chlorine between 1-3 ppm.
– Shock pool with chlorine weekly or as needed to control algae and contaminants.

Use Algaecides

– Apply algaecide at start of season or signs of algae growth.
– Quaternary ammonium algaecides provide good prevention.
– Some contain copper, silver, or zinc to kill existing algae.
– Follow product labels and reapply as directed.

Reduce Nutrients in Water

– Keep phosphates below 100 ppb (0.1 ppm).
– Add phosphate remover if level is high.
– Prevent contaminants like fertilizer and leaves from getting into pool.

Maintain Proper Water Balance

– Ensure pH 7.4-7.6 and alkalinity 80-120 ppm.
– Adjust minerals like calcium if too low or high.
– Perform regular shocking and filtration to strip away organics.

Clean Pool Thoroughly

– Brush entire pool surface at least once a week.
– Vacuum debris from floor thoroughly and often.
– Use pool clarifier if water is cloudy.
– Replace or clean filter media regularly for optimal filtration.

Treating Existing Algae in Swimming Pool

If algae takes hold in a swimming pool, take these steps to get rid of it:

1. Shock the Water

Kill some existing algae and oxidize organics by shocking the water with either:

– Chlorine shock like lithium hypochlorite at 2-3x regular dosage
– Non-chlorine shock like potassium peroxymonosulfate at normal dosage

2. Scrub All Surfaces

Use a stiff pool brush to scrub the entire pool surface thoroughly. This removes some algal growth and biofilms. Pay extra attention to corners and walls prone to algae.

3. Vacuum the Pool

Vacuum the entire floor to waste to remove detached algae and other debris before they decompose and release nutrients back into the water. Clean pump filter afterward.

4. Apply Algaecide

Use a broad spectrum algaecide per label instructions to kill both free-floating and wall-clinging algae. One with copper, silver, or quaternary ammonium works best.

5. Adjust pH and Alkalinity

Test and restore proper pH 7.4-7.6 and total alkalinity around 100 ppm. Proper water balance makes sanitizer more effective against remaining algae.

6. Run Filter 24/7

Run your pool filter continuously for at least 24 hours. This helps remove all the dead algae and waste stirred up by shocking, scrubbing, and algaeciding the pool.

7. Repeat as Needed

It can take several rounds of treatment to fully clear an aggressive algae bloom. Shock, scrub, vacuum, add algaecide, backwash filter, and test chemistry every few days until algae is gone.


Allowing algae growth to get out of control can cause high pH and water chemistry headaches. The photosynthesis and decomposition of algae removes CO2 while adding ammonia to pool water, both increasing pH. Preventing algae in the first place through chlorination, algaecides, and water balance minimizes pH spikes. If faced with a severe algal bloom, vigorously shocking, scrubbing, and vacuuming the pool while using algaecides and muriatic acid can help regain control and lower pH. Staying vigilant to restrict nutrients and maintaining proper sanitizer levels, even after algae is gone, will help keep the pool consistently sparkling and algae-free.

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