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Pool Shock Basics

When it comes to pools, chlorine is mostly considered a good guy. It helps prevent the growth of algae and keeps your pool water safe by neutralizing harmful bacteria and microorganisms. But, as can happen in life, good things sometimes go bad.

When chlorine attaches itself to harmful elements, action must be taken to protect the health and safety of your pool.

That action is shocking the pool.

Why Shock Your Pool?

There are several reasons to shock your pool. Chlorine levels can fluctuate depending on bather 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 is what gives your pool that chlorine smell and can irritate skin, nose, and eyes.

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

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 the Total chlorine results should be equal.

When 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.

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.

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 fight off contaminants over the long winter months.

Algae Outbreaks: When algae appears in your pool, super-chlorination with powerful types of shock can often eliminate the problem.

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. Shocking after pool parties or other unexpected contamination is highly recommended.

Which Shock Should I

There are three types of shock:

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

DIY TIP: To ensure 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 local 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 can destroy the toughest organic and inorganic contaminants and is fast acting. It is a great choice for clean up after a pool party or to destroy algae blooms.

Dichlor pool shock is a fast-dissolving solution and 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 UV rays. Weekly use of Chlor Brite supports the sanitizing efforts of both tab and salt sanitized pools.

Chlorine-Free pool shock is great for quickly oxidizing contaminants, but it will not kills germs or algae. This type of shock will improve water clarity, while complementing enzyme treatments. A non-chlorine variety 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 Total Chlorine the same level; it works great in pools sanitized with liquid chlorine as well.

How to Shock a Pool

Leslie's Water Balancing


Leslie's Pool Circulation


Leslie's Pool Shock


Leslie's 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 to run for about 6-8 hours and then test the water. The Free Available Chlorine level may be very high, making it unsafe to swim.

Step 4. Enjoy Clean, Safe Water

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.

Shock your pool weekly to free up Available Chlorine, which saves money on maintenance products by helping them work efficiently, effectively, and for longer. Shock when you need a quick solution to clear contaminants in the water, when you have a high bather load, or for clearer, fresher water on a regular basis.

At Leslie’s, we’re here to help you create a safe, sanitary, and perfect pool experience.

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