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Pool Shock Basics

When it comes to pools, chlorine is a crucial part of maintaining healthy water. This all-important chemical helps prevent the growth of algae and keeps water safe by neutralizing harmful bacteria and microorganisms. As chlorine works its magic in the pool, it attaches itself to harmful elements in the water and forms chloramines, or inactive Combined Chlorine. That’s when action must be taken to free up chlorine molecules and continue sanitization. Regularly shocking the pool protects the health of your pool and the safety of all pool-goers.

This article is intended to address key questions about pool shock, such as why it’s important to shock the pool, when is the best time to shock your pool, how often should you shock your pool, and what is the best shock for pools.

Why Shock Your Pool?

There are several reasons to shock your pool. Chlorine levels can fluctuate depending on user load and the amount of contaminants in the water. Chlorine attacks and kills germs — which include bacteria and viruses — in your pool water, and, in the process, binds to ammonias from swimmer wastes and other contaminants such as fertilizers, bird droppings, dirt, and debris.  Excessive amounts of these contaminants form Combined Available Chlorine or chloramines, which gives your pool that chlorine smell and can irritate skin, nose, and eyes.

When you use pool shock, you are raising the chlorine level very high for a short period of time to break down the chloramines. This process releases the Free Available Chlorine (FAC) in your water. This allows your primary sanitizer to work efficiently and effectively to keep your pool clean and clear.

DIY TIP: When pool or spa water is tested with a DPD test kit, there are three chlorine results that can be determined. Most colorimetric DPD test kits can test for Free Available Chlorine and Total Available Chlorine. Ideally, the Free and Total Available Chlorine results should be equal. When they’re not, the difference between the two levels determines the amount of Combined Chlorine in your pool.

When is the Best Time to Shock Your Pool

Applying shock to your pool should be part of your regular weekly maintenance, but here are examples of when it's especially important to shock your pool.

Opening & Closing: When opening your pool, it's recommended that you shock your pool after balancing the other chemicals. This will oxidize the particles, kill germs, and clarify the water. When it's time to close your pool, shocking will help prep your pool for the long winter months and fight off contaminants.

Rain & Thunderstorms: During storms, your pool is likely to be contaminated by rain water, dust, and other debris. Shocking after severe weather will help clean up after Mother Nature.

Algae Outbreaks: When algae appears in your pool, super-chlorination with powerful types of shock can often eliminate the problem. Telltale signs of an algae bloom include a green color to your water, and a layer of green film may form on interior pool surfaces.

Heavy Pool Usage: When pools are used by a large number of people in a short amount of time, the water can become contaminated in a variety of ways. Using pool shock after pool parties or a large number of pool users, for example, is highly recommended.

Which Pool Shock Should I Use?

When deciding which type of shock is best for your pool, keep a few things in mind. There are three types of shock:

  1. Cal-Hypo (Calcium Hypochlorite)
  2. Dichlor (Dichloro-S-Triazinetrione)
  3. Chlorine-Free

DIY TIP: To ensure you're using the best shock for your pool, don’t guess. Always test your water at home or bring it in for a free AccuBlue® test at your nearest Leslie’s location.

Cal-hypo is a powerful and effective shock. It quickly kills algae and contaminants, and is ideal for pools with a normal-to-low pH level. Leslie’s Power Powder Plus 73 is great for overall pool clean-up when chlorine levels are below 2.0 ppm. It is the strongest shock, providing 73% Calcium Hypochlorite and 70% Available Chlorine. Power Powder Plus 73 can be used weekly, but is also known as a solution shock because it is fast-acting and can destroy the toughest organic and inorganic contaminants. It is a great choice for post-party cleanup or to destroy algae blooms.

Dichlor pool shock is a fast-dissolving solution that will not raise the pH level or cloud the pool water. When chlorine levels are below 2.0 ppm you can choose a maintenance shock like Leslie’s Chlor Brite, a stabilized granular chlorine shock that won’t burn off quickly from the sun’s UV rays. Weekly use of Chlor Brite supports the sanitizing efforts of both traditional chlorine tablets and saltwater chlorine generator systems.

Chlorine-free pool shock is great for quickly oxidizing contaminants, but it will not kill germs or algae. This type of shock will improve water clarity, while complementing enzyme treatments. A non-chlorine variety of shock such as Leslie’s Fresh 'N Clear, is perfect for weekly use when FAC levels are in the 2.0-4.0 ppm range. Fresh 'N Clear oxidizes and destroys ammonia and non-living organics, clears cloudy water, and helps keep your FAC and TAC at the same level. It works great in pools sanitized with liquid chlorine, as well.

How to Shock a Pool

Leslie's Water Balancing

Test the Water

Leslie's Pool Circulation

Circulate the Water

Leslie's Pool Shock

Pour the Shock

Leslie's Clean Water

Enjoy Clean Water

Step 1. Test the Water

Use a water test kit or test strips to ensure that the pH level is in the proper range.

Step 2. Circulate the Water, Pour the Shock

Turn on your filter pump to circulate the water. Now, pour the shock product around the inside perimeter of your pool. The best time to shock is at dusk or night, and then let your filter pump run overnight.

Step 3. Wait for FAC Level to Drop

Continue letting the pump run for about 6-8 hours, and then test the water. The FAC level may be very high, making it unsafe to swim. Wait until the FAC level falls to 2-4 ppm before jumping in. Keep in mind that a cal-hypo based shock, such as Power Powder Plus 73, contains a small amount of calcium and can potentially cloud the water for up to 12-24 hours in certain situations.

Step 4. Enjoy Clean, Safe Water

Shock your pool weekly to free up FAC levels, which saves money on maintenance products by helping chlorine work efficiently, effectively, and for longer. Shock when you need a quick solution to clear contaminants in the water, when you have a high user load, or to achieve clearer, fresher water regularly.

You can rely on the team of experts at Leslie’s. We’re here to answer all your toughest pool shock questions. like when is the best time to shock your pool, how often should you shock your pool, and what is the best shock for certain pool situations. Call or stop by your local Leslie’s for more information.

Top Products

Leslie's Power Powder Plus Cal-hypo Shock

Calcium Hypochlorite

Chlor-Brite Dichlor Chlorine

Sodium Dichlor

Leslie's Fresh N'Clear Non-Chlorine Shock


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