How to Lower pH in a Pool Naturally

Reviewed by Andrada Simion, Master of Science in Chemistry

“Naturally” can mean different things to different people, so this article is going to cover a range of methods including “non-chemical” ones (though technically speaking even pure water is a chemical) and use of household products (and we recommend against most of them, read on to find out why).

Rainwater

Rainwater ranges from pH 5.6 for clean rain all the way down to 4.2 for acid rain, so you could add water from a collection tank or even channel water directly from roof gutters (or if it’s a rainy area or a wet season take/leave the pool cover off, while making sure you don’t compromise on safety or run afoul of local pool safety laws).

Rainwater will also lower total alkalinity, calcium hardness, chlorine and cya levels (any of these that were too high to begin with would see a benefit but if not they will need to be replenished), some passively through simple dilution and some also actively from organic and other debris – even if you manage to avoid or remove visible ones like leaves and twigs there will be lots of dissolved matter that could cause more problems than merely lowered levels of chemicals.

Carbon Dioxide (CO2)

That’s right, the very gas produced by the cells in our bodies and exhaled through each and every breath (to the tune of 2.3 pounds a day), lowers pH when dissolved in water (which is why carbonated water is acidic). However just as sodas go flat after being opened or poured, any added CO2 will escape into the air over time which is why injection systems that automatically replenish it are preferable to the other method of simply tossing in some dry ice. You can reduce CO2 loss by using pool covers and by keeping temperatures low.

Some of the pros of this method are:

  • it allows controlled and precise changes,
  • no need to handle or mix more dangerous chemicals
  • alkalinity isn’t affected unlike with other pH reducers
  • environmentally friendly (more details in the conclusion section at the bottom)

However its cons include:

  • higher (especially upfront) costs
  • CO2 loss will be too quick in pools with lots of movement like fountains and spas
  • if calcium or alkalinity is high then it will be far less effective

Household Products

Spoiler: Avoid them and just use muriatic acid, whose main ingredient is HCl (which is “natural” considering that it’s produced by your own stomach – this doesn’t mean it’s safe for consumption though!).

Vinegar (diluted acetic acid) is much weaker than muriatic acid (read: less effective at lowering pH) so you’ll need to use around 10 times as much for the same effect (read: more expensive and time-consuming). It will also stink to high-heaven, but that would be the least of your worries – it would react with other chemicals in the water forming harmful substances like haloacetic acids (which may increase the risk of cancer and birth defects), and if you use naturally brewed vinegar containing all sorts of other impurities you’d end up with an even wider variety of god-knows-what harmful byproducts.

Citric acid is also found in many households, usually used as a cleaning agent but is also found in commercial food products as well as in lemons and other citrus fruits (hence its name). It’s even weaker than vinegar so you’d need even more of it, plus it will react with and greatly reduce chlorine levels while creating trichloromethane (aka the toxic and carcinogenic chloroform) and other toxic byproducts.

Conclusion and Environmental Friendliness

Chemically speaking, most of the above methods are no more “natural” than some commercial pool products and are worse in many ways. If you’re trying to be environmentally friendly, muriatic acid’s only byproducts after being added are water and sodium chloride (aka plain salt) – better than sulfuric acid, a commonly used alternative which forms harmful and environmentally-unfriendly (not to mention stinky) sulfates, but inferior to CO2 which forms carbonates/bicarbonates that are less corrosive than sodium chloride (and while we’re on the topic of the environment, research has found that chlorine usage, oxidant emissions and trihalomethane emissions were reduced by using CO2 in pools). In addition, carbon dioxide outgassing isn’t reduced by using a different type of acid either (you do that by keeping total alkalinity low).