How to Lower Alkalinity in a Hot Tub (Step-by-Step Guide)


Keeping alkalinity levels within the correct range is vitally important for the safe running of your hot tub and its components. So how do you lower the alkalinity?

In order to lower the alkalinity of hot tub water, you’ll need to add acid. The most practical and safe choice is sodium bisulfate, which is sold as alkalinity decreaser. An alternative way to lower alkalinity is to use muriatic acid. Alkalinity cannot be lowered naturally without the use of chemicals.

In this article, I show you:

  • The two best ways to lower alkalinity
  • The damaging effects of high alkalinity
  • The cause of high alkalinity
  • How to prevent it from reoccurring

Two ways to lower alkalinity in a hot tub

Make sure you know the capacity of your hot tub before making adjustments to the total alkalinity (TA) so that you can accurately calculate the dosage.

The optimum range for total alkalinity is often stated as being between 80 – 100 parts per million (ppm). However, a reading between 100 – 150 ppm is recommended as it gives you some breathing room should things change unexpectedly.

How to lower alkalinity using sodium bisulfate

The easiest way to lower alkalinity levels in a hot tub is by using sodium bisulfate, which is sold as Alkalinity Decreaser. Sodium bisulfate is a dry acid in granular form that’s user-friendly.

Despite being safer than liquid acid, caution still needs to be exercised when handling any type of acid by wearing protective gear.

Note: As well as lowering alkalinity, sodium bisulfate also lowers pH, which is why it’s also sold as pH Decreaser. There’s no need to buy both Alkalinity Decreaser and pH Decreaser because they’re the same thing.

What you’ll need:

How to lower alkalinity using sodium bisulfate

  1. Using your liquid test kit or test strips, take a reading of the current alkalinity level.
  2. Follow the instructions on the bottle to calculate how much sodium bisulfate you’ll need.
  3. Put on rubber gloves and goggles.
  4. Fill a clean bucket 3/4 full with water drawn from the spa. Add the sodium bisulfate to the bucket and stir until completely dissolved.
  5. Turn on the pump and jets.
  6. Pour the solution into the middle of your hot tub and allow it to circulate for four hours with the cover removed.
  7. Retest and adjust if necessary.

When sodium bisulfate is added to water, it increases the hydrogen concentration, lowering both the total alkalinity and the pH. The exact amount of sodium bisulfate your spa needs will differ each time you have to lower alkalinity levels.

As a general rule of thumb, you’ll need to use 1/3-ounce (9.4g) of sodium bisulfate per 100 gallons (380 liters) of spa water. A 1/3-ounce of sodium bisulfate in 100 gallons will lower alkalinity by approximately 10 ppm. Keep in mind that sodium bisulfate will also lower the pH levels.

ProductDosage (imperial) Dosage (metric)
Sodium bisulfate1/3-ounce per 100 gallons water9.4g per 380 liters water

After adding sodium bisulfate, you’ll need to wait somewhere between two to four hours while it circulates. Some manufacturers recommend running a full cycle before retesting. The more pH decreaser you add, the longer you should wait before retesting.

Keep the cover off whenever you add chemicals so that the water can breathe. Releasing obnoxious chemical odors also prevents damage to the vinyl cover.

It’s important to always add sodium bisulfate to water and not the other way around. While sodium bisulfate is less caustic than liquid acid products, the undissolved acid granules can still damage surfaces such as the hot tub shell if it’s accidentally spilled.

PH decreaser can be stored in a wider range of conditions, making it far more practical than liquid acid. However, because it’s a dry acid, do not allow any moisture to enter the container. Store it in a cool, dry place out of reach of children.

How to lower alkalinity using muriatic acid

Liquid muriatic acid (or Hydrochloric acid) is an alternative way to reduce the alkalinity in hot tubs. Popular in commercial settings, muriatic acid is both cheaper and more effective than dry acid. It’s also able to kill mold and remove calcium deposits.

Extreme care needs to be taken when handling muriatic acid as it can result in respiratory problems, burns, and blindness. Only handle muriatic acid if you feel completely confident in your ability to do so.

What you’ll need:

How to lower pH using muriatic acid:

  1. Use a liquid test kit (most accurate) or test strips (less accurate) to obtain a reading of the current alkalinity level.
  2. Consult the directions on the bottle to determine the amount of muriatic acid you need to lower the alkalinity.
  3. Put on your protective gear: long-sleeve gloves, safety goggles, chemical mask, and vinyl apron.
  4. Using a clean bucket, dilute one part acid into 10 parts water. Mix the solution thoroughly using a wooden or plastic stirrer.
  5. Turn on the pump and very slowly pour the solution into the center of the hot tub.
  6. Allow the solution to circulate for at least one hour.
  7. Retest and make adjustments if necessary.

The exact amount of muriatic acid your spa needs will differ each time you have to lower alkalinity levels.

As a general rule of thumb, you’ll need to use 2.6 fluid ounces (77ml) of full-strength muriatic acid (31.45% hydrochloric acid) per 100 gallons (380 liters) of spa water. A 2.6 fl oz dose of muriatic acid in 100 gallons will lower alkalinity by approximately 10 ppm. Remember that muriatic acid will also lower the pH levels.

ProductDosage (imperial) Dosage (metric)
Muriatic acid2.6 fluid ounces per 100 gallons water77ml per 380 liters water

Muriatic acid is highly corrosive, which is why it has to be diluted before adding it to your hot tub. Accidental splashes will damage the spa shell. Muriatic acid is so powerful that it’s capable of burning through flesh, metal, and concrete.

Ensure that you rinse the bucket thoroughly after each use. Never use the bucket for any other chemicals as there’s a risk of causing an explosion.

Because of the risks involved when using muriatic acid, I recommend using a safer alternative called Acid MagicOpens in a new tab.. It has the alkalinity reduction properties of muriatic acid, but it’s less caustic and gives off 90 percent fewer fumes.

Despite the manufacturer stating that it will not cause burns to the skin, you should still wear full protective gear when handling Acid Magic.

Again, keep the cover off when adding chemicals to allow the water to breathe and to prevent causing damage to the vinyl cover.

What happens if alkalinity is too high in a hot tub?

I know how easy it is to become complacent when it comes to routinely monitoring water chemistry. So what are the effects of high alkalinity?

If the alkalinity of your water is too high, it results in an increase in pH. High pH levels greatly reduce the effectiveness of chlorine and bromine sanitizers, resulting in a build-up of harmful bacteria. High pH poses risks to bathers’ health and results in a build-up of limescale that damages the hot tub.

When the water is within the correct TA range, it acts as a buffer that takes the hit so that there aren’t fluctuations in the pH.

[Alkalinity is] a measure of the ability of the water body to neutralize acids and bases and thus maintain a fairly stable pH level

usgs.govOpens in a new tab.

If the alkalinity level of your spa water is allowed to get too high, it causes stubbornly high pH levels that lead to a whole host of problems. One of these problems is the negative effect high pH has on sanitizing chemicals.

Your spa’s pH should be between 7.4 – 7.6. Chlorine is barely 20 percent effective in water with a pH of 8.0. That figure reduces to under 10 percent if the water is allowed to reach a pH of 8.5 or higher.

The decrease in effectiveness results in you wasting money by adding more and more chemicals in an attempt to achieve the same sanitizing effects. Because the sanitizers can’t do their job properly, the spa water can quickly turn green as a result of algae blooms.

As bacteria levels rise, bathers may experience allergic reactions, gastrointestinal upsets, stomach cramps, nausea, vomiting, and breathing difficulties. Soaking in high pH water will also cause itchy skin, burning eyes, and nose and throat irritation.

A rise in pH also results in high levels of calcium, which turns the water cloudy and causes white chalky flakes to float on the surface of the water.

Limescale deposits cause significant damage to components such as the pump and heater, which are very costly to replace. The scale will also clog the spa’s jets and filter, which can greatly decrease the flow of water and increase heat-up times.

Why is the alkalinity high in my hot tub?

You might be baffled as to why your spa water has such high alkalinity levels if you routinely keep on top of balancing the water. Here are the reasons why you’re experiencing high alkalinity.

High alkalinity is caused by adding too much alkalinity increaser, too many chemicals, and contaminating the water with bodily fluids and body products. The water hardness level in your area may mean that the water source has a naturally high alkalinity level.

The alkalinity level in your hot tub may be too high because you’ve added too much baking soda or soda ash (alkalinity increaser) in an attempt to raise alkalinity levels.

If you own a swimming pool and you’ve used liquid chlorine (sodium hypochlorate) intended for pools in your hot tub, this could be another reason for high alkalinity. The alkalinity rises due to excess hydroxides left behind by liquid chlorine, which is why it should only ever be used in pools.

Bodily fluids and beauty products play havoc with the water chemistry. Always take time to take a quick shower to rinse off contaminants from your body before you get into your hot tub.

Rinsing off is great practice; however, no matter how much you scrub, the fact is that you’ll always introduce bacteria to the water each time you take a soak. This is why we have to add chemicals that kill bacteria and prevent a build-up.

Unfortunately, spa water can only take so many chemicals before it becomes “full” and the chemicals no longer have any effect.

Think of adding salt to a glass of water. To begin with, the salt dissolves. But as you add more and more, the water is no longer able to dissolve the salt. This is what happens over time as you keep adding chemicals to your spa water.

To combat this, ensure that you’re draining all of the spa water every three to four months to keep things fresh.

Lastly, the culprit may be your water supplier. The amount of alkalinity in drinking water is usually between 20 – 200 ppm, so it might be that you live in an area of the country where the water source has a naturally high alkalinity reading.

If messing about with water chemistry seems like a hassle, I recommend you use Silk BalanceOpens in a new tab.. Once you have the alkalinity and pH levels set, Silk Balance keeps it locked so that you don’t have to waste time monitoring and adjusting it.

Did I cover everything?

I always try to provide the most relevant and up-to-date information I can in all of my articles. Saying that, feel free to shoot me an email using the contact form if you think this article is lacking in some way, or if you’ve been left with any doubts.

Thanks for reading and happy hot-tubbing!

Did I cover everything?

I always try to provide the most relevant and up-to-date information I can in all of my articles. Saying that, feel free to shoot me an email using the contact form if you think this article is lacking in some way, or if you’ve been left with any doubts.

Thanks for reading and happy hot-tubbing!

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