Pool pH & Alkalinity for Stain & Algae Prevention

drpoolMy esteemed colleague Melissa wrote, in her article on pool staining in a marcite pool, “Pool stains don’t just happen, they are caused by poor water conditions!”. I concur.

Today’s lecture investigates the relationship between the relative acidity of pool water, and the causal effect of pool stains and algae growth.

My premise is this – proper pool pH and alkalinity levels, will reduce the potential for pool staining and the growth of algae.

pH and Alkalinity

Let’s begin with a review of the basics. pH is a scale of how acidic or how basic a substance is, and every solution and substance, natural and man made, has a pH value somewhere on the continuum of acid and alkalinity.


The best pH for swimming pools and spas is a range of 7.2 – 7.6. Just above neutral 7.0, slightly alkaline on the pH scale.

Total Alkalinity is a measure of your pools potential to neutralize acids; and acts as a buffering agent, or stabilizer for the pH level. Low alkalinity levels will cause the pH to become erratic and hard to stabilize, while high alkalinity has the opposite effect and makes the pool pH hard to adjust.

perfect-pool-alkalinity-2Your best level of total alkalinity is usually in the range of 80-120 ppm. Pool pH is usually easiest to control when the pool alkalinity is hovering close to 100 ppm.

Pool Stains – Pool pH – Pool Alkalinity

Low pH environments can lead to pool staining from metals and minerals. If you have copper pipes or a gas pool heater, consistently low pH can erode soft copper metals and contribute to staining.

NOT the MOON - but actually a pool step with both etching and scaling, the result of poor pool pH managementIn plaster pools and also in vinyl pools, a low pH environment can damage pool surfaces, making them more susceptible to staining from minerals and organic materials.

High pH and alkalinity, coupled with high Calcium Hardness levels, can cause scaling to occur, which mixed with organics (dirt and oil), deposits itself on your pool surfaces or at the water line, as a stain.

To prevent pool stains, you’ve got to keep pool pH and alkalinity within their proper ranges, neither too high, nor too low. Do this one simple thing, and the possibility for pool staining drops to a much lower level.

Pool Algae – Pool pH – Pool Alkalinity

Phytoplankton, or pool algae prefers a higher pH of 8.0 or above, but to thrive it also needs a steady source of sunlight, nutrients and minerals.

High pool pH won’t necessarily cause algae growth per se, but it does have the effect of reducing the efficacy of chlorine by up to 80%. Chlorine is most effective in the lower ranges of pH, principally between 7.2 – 7.6, and higher pool pH reduces chlorine strength considerably.

Algae blooms however, can raise the pH of your pool water. The process of photosynthesis can produce enough oxygen in the water to raise your pool pH levels, particularly if tested near the end of a warm, sunny day.

ph-decreaser for pool pH and alkalinity reductionHigh pool alkalinity levels will make it hard to adjust your pH level. To lower your total alkalinity, use pH decreaser. In most cases, additional treatments will be necessary if you need to lower alkalinity more than 30 or 40 ppm.

There are pools in some parts of the country that have very alkaline tap water, of 300 ppm or more. To lower such high alkalinity can take 3 or 4 consecutive treatments, so that the pH level doesn’t sink below 7.0 neutral, creating an acidic environment.

Lowering pool alkalinity should be done carefully, and in stages, to prevent the pool pH from going too low; potentially damaging pool surfaces or equipment.

Keeping it all in Balance


To prevent pool stains, keep pH and alkalinity balanced, to avoid mineral stains (from low levels), and scaling stains (from high levels).

For algae prevention, proper pool pH and alkalinity will allow your sanitizer to work more effectively; which lowers your chlorine cost, or reduces the workload asked of a salt chlorinator.

Remember, Pool Stains and Pool Algae don’t just happen, they need certain water conditions to occur. Take care of your pool’s pH and alkalinity first – and problems with staining or algae will be easier to manage.



Dr. Pool


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