How To Shock Your Hot Tub Or Spa (And When You Should)



With a high bather load in a small body of water, hot tubs are subjected to high levels of pollutants every time they’re used.

While their water chemistry contains a sanitizer such as chlorine or bromine, if the level of contaminants in the water exceed the level of sanitizer, you’ll be bathing in filth.

Shocking a hot tub refers to dosing the tub with a massive concentration of sanitizer, in order to kill off any viruses, bacteria, and pathogens that the day-to-day sanitizer can no longer eradicate.

Why Shock Your Hot Tub?

Shocking a hot tub provides spa owners multiple benefits to keep their tub operating at peak performance.

  • It clears cloudy water. Hot tub water is supposed to look inviting and crystal clear. If the water begins to get cloudy, either from pollutants, or from too much calcium in the water, a surefire way to clear it up is by adding a dose of shock to the tub.
  • Say goodbye to chloramines. Chloramines (combined chlorine) are gassed-off pollutants in the water that give a hot tub that pungent chlorine smell. When the tub smells like this, it actually means there’s not enough sanitizer in the water. Bromine tubs are similar but they produce bromamines instead. Shocking the tub will eliminate both as well as the nasty smell that comes from them.
  • It battles bacteria. Unclean bodies of water, especially high temperature ones like a spa, are breeding grounds for bacteria, which can make bathers sick. Some common illnesses caused by bacteria are folliculitis (skin problems), Legionnarie’s disease (severe pneumonia), and non-tuberculous mycobacteria (chronically inflamed lung disease).
  • It fights organic contaminants. Bathers bring in a lot of organic contaminants like body oils, skin flakes, makeup, sunblock, and even shampoo and swimsuit detergent residue. These quickly use up the water’s sanitizer, which is why public pools and spas require you to shower before use. At high amounts, the sanitizer struggles to keep up and shocking is the only way to get rid of them.
  • It helps reset sanitizer levels. Once the sanitizer is overloaded by pollutants, it becomes ineffective and needs a helping hand. Shocking helps out by killing everything in its path via a massive dose/concentration of sanitizer. This allows you to reset the day-to-day sanitizer level and start the cycle over. Without shock, you’d have to continually add massive amounts of everyday sanitizer, which is unrealistic and costly.

When Should You Shock?

With hot tubs, cleanliness is king, and this should be your number one priority so that people don’t fall seriously ill after going in the water.

Most owners follow a strict maintenance routine for exactly this reason. Routine cleaning also makes it simple for you to stay on schedule, ensuring your water is working at its best 24/7.

Shocking is done once a week, but if you use the tub often (and with multiple bathers), you may have to shock more frequently.

It’s typically done overnight, as shock has the best chance of eradicating everything in the water when it’s not exposed to sunlight.

The issue with shock is that both chlorine and bromine are chemicals that are destroyed within hours by the sun’s UV rays.

For this reason, most indoor or shaded hot tubs use bromine, while tubs in direct sunlight use chlorine with cyanuric acid (CYA). CYA acts as a sunblock for the chlorine so it lasts longer. Unfortunately there is no “sunblock” additive for bromine, which is why these tubs must stay out of sunlight.

By shocking overnight, it takes the sun out of the equation. The shock can kill nasty things for 8+ hours, and will then burn off once the sun comes out. 

What Type Of Shock Should You Use?

There are a few different types of shock available for use. Let’s take a close look at which ones you can use for your tub.

Important: You can use chlorine shock in either chlorine or bromine hot tubs, just don’t mix chlorine shock and bromine at the same time. Add the shock, wait for the chlorine level to go down, and then add in your bromine sanitizer.


Sodium dichlor is the best pool shock to use for chlorine or bromine hot tubs.

It has a 55% concentration of chlorine and is stabilized (contains CYA), giving it some extra life in your hot tub.

The downside is that it contains CYA, which will cause the CYA levels in your tub to go up the more you use it. However, as long as you closely monitor this level, you shouldn’t have a major issue with it.

Calcium Hypochlorite

Commonly referred to as cal hypo, this chlorine shock has a high chlorine concentration of over 70%, which, in theory, would make it an ideal shock.

It’s also unstabilized (no CYA) which means it will burn off quickly once the sun hits the tub. However, cal hypo also contains a lot of calcium, which is why it’s not typically the best shock for your spa.

Once the calcium levels spike, you’ll either have to do a drain, or use a flocculant to remove the calcium from the water.

Lithium Hypochlorite

Lithium hypochlorite is a granular shock with 28% to 35% chlorine, but it’s rarely available anymore due to the increasing demand of lithium for battery production. 

At one point it was a fairly popular shock due to its ease of use, high solubility, and zero calcium content. Today, it’s almost impossible to find, and it’s likely to be very expensive even if you do.

Non-chlorine shock

Added weekly, MPS shock (potassium monopersulfate) is an oxidizer, and not a disinfectant, meaning it will only tackle the organic pollutants (body oils, skin flakes, sunblock, etc.) found in the water.

It’s role is to free up the chlorine/bromine in the tub so it can be more effective. If used in a chlorine tub, non-chlorine shock breaks apart combined chlorine molecules, restoring the free chlorine so it can continue cleaning the water. 

Bromine, on the other hand, can regenerate. Non-chlorine shock will oxidize the spent bromine ions and create new bromine in the water.

How To Shock A Hot Tub Or Spa

Always remember to practice proper safety when working with shock, as high concentration chlorine can bleach you or the tub if you’re not careful.

You should also be aware that the shock fumes can irritate your nasal passage and lungs if inhaled.

What you’ll need for shocking:

  • Test strips or a testing kit
  • Chemically resistant gloves
  • Protective eyewear
  • A chemical mask
  • A plastic measuring cup
  • A 5 litre plastic bucket
  • A wooden stir stick

Step 1. Remove The Cover

First things first, remove your hot tub’s cover so you have access to the water.

Place it somewhere it won’t get damaged if you accidentally spill chlorine. Turn off the pump so there’s no water coming through the jets and the water is still.

Step 2. Balance The Water

Next up is testing and balancing the water chemistry. Break out your test strips or testing kit and make note of the pH, total alkalinity (TA), and calcium hardness (CH). 

For pH, you want it between 7.4 and 7.6, TA between 80 and 120 ppm, and CH between 175 and 250 ppm.

Add in the proper chemicals needed to balance these levels and then move on to the next step.

Step 3. Get Your Measurements

From testing, you should know how much chlorine or bromine is currently in the water. In chlorine tubs, you need to raise the combined chlorine levels by 10 in order to reach breakpoint. 

For example, chlorine must be between 1 and 3 ppm for clean water. If your combined chlorine level tested at 0.5 ppm, you need to add enough shock to raise it to 5 ppm.

In bromine hot tubs, the ideal level is between 3 and 5 ppm. Unlike chlorine, there is no “combined bromine” level.

After the bromine level is tested, if it’s under 3 ppm, you’ll have to shock it with 10 times the level that’s currently in the tub. So If it’s testing at 1 ppm, you’ll have to add 10 ppm of shock.

Step 4. Add It To The Water

Any kind of pool shock should be diluted before you pour it into the hot tub. This is a protective measure because of the chemical’s high concentration. The last thing you want is chlorinated shock to accidentally bleach your tub.

Start by suiting up in your protective gear – gloves, goggles, and a mask. Then, scoop water from the tub using your 5 litre bucket. Measure your shock dosage with a measuring cup. 

Pour the shock into the bucket of water. Always add the chemical to the water, do not add water to the chemical. This can cause it to splash up and you want to avoid accidents.

Stir the shock into the water using a wooden stir stick. Make sure it’s fully dissolved in the bucket of water.

Slowly pour the shock solution into the tub. If you notice any granules floating in the water, brush them off the walls so they don’t damage the shell’s finish.

Step 5. Allow Enough Time (Before Use)

Turn on the tub and allow it to run for at least an hour to circulate the shock. Turn off the tub and retest the water’s sanitizer levels. Adjust them accordingly.

If you’re using non-chlorine shock, you only have to wait 15 minutes before retesting the water. Just make sure your chlorine and bromine levels are where they should be before using the hot tub.

A Shocking Display!

Learning how to shock a hot tub isn’t rocket science, but it does require you understand how both chlorine and non-chlorine shock interact with sanitizer levels in the tub.

As long as you follow a regular maintenance schedule, and increase testing when the tub is used often, you should have no problems keeping your sanitizer in line.

Categories: Hot Tub Care, Hot Tub Chemistry