How To Shock A Swimming Pool

Have you ever taken a look at your pool and said to yourself, "Gee, the water doesn't look right." A pool should have crystal-clear and inviting waters, not murky and sickly looking. If the pool water doesn't look right, then you might have algae growth and this can be a bit of a concern. Not only does algae growth make your pool look unappealing, but it can be dangerous as well, as algae is very slippery and can cause accidents.

On top of that, algae has been known to cause allergic reactions in some people, as well as damage your pool filters and other fittings if left unchecked. So at the first sign of algae growth, take some time to shock your pool and keep your pool waters crystal clear.  

What is shocking a swimming pool, and how do I do it?

  1. Sanitise your pool with a heavy treatment of chlorine (either granular or liquid). Ensure that non-stabilised chlorine is used.
  2. Lower your pH with hydrochloric acid. By doing this, the chlorine will be more effective.
  3. Ensure the system is on and running for a 24 hour period to disperse the shock treatment around the pool and clean the filter for any debris captured.
  4. Test and re-balance 2 to 3 days after and make any additional adjustments required. Switch the filtration system back to its normal operating procedure.

Three stages of Pool Algae Growth

There are 3 different stages of pool algae growth in which different amounts of pool shocking are needed:

  • Light green algae
  • Dark green algae
  • Black green algae

These are not different types of algae, but rather different levels of algae growth. The more algae in your pool water, the darker the green colour will be. We’ve seen pools almost black with an algae infestation. Gross!

Light Green Pool Water

If you have a light green pool, you need to shock your pool (adding more acid and chlorine than usual) in order to kill the algae.

Algae feeds off high pH. Chlorine isn't as effective when your pH is high. You’ll need to lower your pH using hydrochloric acid. Start with around 1L to 2L. Be careful when adding it to the pool, ensuring it doesn't splash up on you as it is highly toxic and dangerous. If your pool is vinyl lined, only add 1L every 24 hours, as it's more sensitive to harsh chemicals.

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You also need to shock your pool with chlorine. Add 2 cups (400 grams) of granulated chlorine (not stabilised) or 3 tubs of 15L chlorine (as it's actually only 20% chlorine and the rest is water). If your pool is salt water chlorinated, add 2 to 4 bags of salt (once again you should get your salt tested at your local pool shop to see exactly where it's sitting) and turn your chlorinator to 100% for a couple of days to really boost that chlorine and get that water sanitised. Your salt levels should be sitting at around 5000ppm (parts per million), with each 20kg bag of salt equating to 500ppm.

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*We recommend only using stabilised chlorine (featured above) if absolutely necessary. Preferably, you should be using non-stabilised chlorine and adding in your stabiliser separately to avoid a chlorine lock (put simply, chlorine lock means that the chlorine in the pool is useless which means the water isn't being sanitised. It can also indicate the presence of chloramines which give off a chlorine smell).

The reason for this is because you require much less stabiliser than you do chlorine.This issue can be avoided as a whole by installing a chlorinator if you don't already have one."

If you've added your salt and turned your chlorinator up to 100% but, after a few days, there's still no ( or next to none), chlorine being produced or your "low salt" light is still on, your electrode may need replacing. The average lifespan of an electrode is 3 to 5 years.

You also will need to clean your cartridge filter or backwash your sand filter and ensure your skimmer baskets and pump baskets are empty.

If the green in your water has made its home on your pool steps and doesn't want to budge, adding hydrochloric acid should slowly start killing the algae. If your pool is concrete, you can use a metal brush to help brush this algae off, or if your pool is vinyl you can use a normal plastic bristle brush.

Mr Pool Man can't sell or ship hydrochloric acid, liquid chlorine, granulated chlorine or salt online as it's either dangerous or too heavy. You can grab them from your local pool shop.

We recommend testing your water a few days after this heavy dose to check your levels. You can do this yourself using a 7 in 1 Test Strip Kit.

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Once your pool is sparkling clean, don't forget to add 500g to 1kg of stabiliser/cyanuric ccid/UV blockout to keep the chlorine in the pool, preventing it from being sucked up by the sun. Adding 500g of stabiliser once a month should keep this in check.

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Dark Green Pool Water

If you have dark green pool water, you should look to follow the above steps, but with a slightly higher dose of acid and chlorine. Adding, for example, 2 to 3 litres of acid over a space of 48 hours and adding 2 to 3 cups (600g) of granulated chlorine. Also, darker green water may also be a cause of stagnant debris sitting on the bottom of the pool which needs to be cleaned out before you dose it. Be sure that this has all been scooped up using a leaf shovel, followed by being manually vacuumed out of the pool using a vacuum head, an appropriate length hose for your pool (9m, 11m or 15m) and a telepole.

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Black Green Pool Water

If you have black green pool water, you will be looking at doing the ‘Light Green Pool Water’ dosage as outlined above x3. This may take some filtering, cleaning or backwashing of the filter, and more filtering to completely clean up over a period of 2 to 3 weeks.

Black green pool water will almost certainly leave stagnant debris sitting on the bottom of the pool which needs to be cleaned out before you dose it. Whether it be leaves or other debris, be sure that it is all scooped up using a leaf shovel, before being manually vacuumed out of the pool using a vacuum head, an appropriate length hose for your pool (9m, 11m or 15m) and a telepole.

Note: If you have a vinyl lined pool, diluting any chemicals that are being added to your pool in SEPARATE tubs of water (we don't want any chemical reactions occurring) is always a good idea to ensure no damage is done to the liner.

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Please see a list of other useful pool blogs below:

Everything you need to know about Pool Pumps.

Our Collection of Pool Filters and how to choose the best one.

Every single listing of Pool Chemicals we carry and how to best utilize them.

All of our Pool Maintenance articles in one place.

Note: This blog is a rough indication of advice for an average sized pool and average dosing recommendations. Mr Pool Man always recommends getting your water tested by a pool professional before adding any type of chemicals for your own safety or contacting us if you are unsure. Mr Pool Man does not take any responsibility for incorrect dosages

Do you have any questions about this topic or the featured products? No worries, we're here to help! Drop us a question down below and we'll get back to you ASAP.

Happy swimming :)

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Post author:

Tom Hintze

Head of eCommerce & Operations at Mr Pool Man,
Co-Founder at Water TechniX


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