An Overview of Pool Shocking

Someone pouring pool shock into a pool.

Pool shocking doesn’t have anything to do with electricity – it’s just the term for adding chemicals like chlorine into your pool to kill pathogenic bacteria left in the water after bathers go swimming in it. It got its name from shock, the kind of chlorine used for pool maintenance. Aside from bather waste (skin, hair, lotions, cosmetics, soaps, fungus and sweat), pool shocking also removes: 

  • Algae – Algaecides can help control the growth of algae. But to completely get rid of them and clear your pool, chlorine – also known as the most potent algaecide – needs to be applied. To maximize the shock’s algaecide effect, it should be applied when the water’s potential hydrogen (pH) level is between 7.1 to 7.3. Depending on the severity of the algae problem, 10 to 30 parts per million (ppm) of shock will be needed to kill all algae blooms. 
  • Chloramines or combined chlorine – There are two types of chlorine in your pool: free chlorine and combined chlorine (also known as chloramines). The former oxidizes and destroys organic material in the pool while the latter has lost its potency after bonding with ammonia or nitrogen. Once chloramines have already done their job, they should be neutralized. That’s because they can cause skin and eye irritation, and produce a strong chlorine smell. In high enough levels, combined chlorine can be toxic to swimmers. To remove the chloramines in your pool, you’ll need to add free chlorine that’s 10 times the amount of combined chlorine in the pool. 

Chlorinated vs. Non-Chlorinated Shock 

There are also two types of shocks: the chlorinated and non-chlorinated variety. Non-chlorinated shock (which is made from a combination of oxygen and potassium) can’t eliminate pathogenic bacteria or algae. As such, it should only be used when the pool water starts to turn murky or the amount of chloramines is too high. As a rule of thumb, non-chlorine shock should be used when chlorine levels are between 2 to 4 ppm. Chlorinated shock, on the other hand, oxidizes and destroys both organic materials and combined chlorine. 

There are three different types of chlorinated shock. All three should be able to clear up your pool, but there are a few differences between them. Here’s an overview: 

  • Liquid chlorine – Of the three, liquid chlorine has the highest pH level. It should be used only in low doses (about 1 gallon per 10,000 gallons of water). That’s because in high amounts, liquid chlorine can bleach paint, vinyl, and other sensitive surfaces. 
  • Calcium Hypochlorite (Cal-Hypo) – Cal-Hypo also has a high pH level, although it’s not as high as liquid chlorine. Like liquid chlorine, it should also be applied in low doses (approximately 1 pound per 10,000 -to15,000 gallons of water). One advantage of Cal-Hypo is that it can increase the levels of free chlorine in your pool without increasing cyanuric acid. Keep in mind this doesn’t mean cyanuric acid is all bad. In small amounts, it acts as a stabilizer by protecting chlorine from sunlight. However, in large amounts, it can cause chlorine lock, which essentially renders free chlorine useless. 

Here’s a pro tip: Cal-Hypo should be applied at night to give it enough time to work. The same rule applies to most types of shocks. 

  • Lithium Hypochlorite – Lithium hypochlorite won’t damage your pool’s vinyl lining or leave any residue. However, it’s a bit on the pricey side and needs to be applied in large doses (as much as 1 pound for every 8,000 gallons of pool water). 
  • Dichlor and Trichlor – A more cost-effective alternative to Lithium Hypochlorite would be dichlor and trichlor. They won’t damage vinyl-lined pools or fiberglass surfaces. And, thanks to their exceptionally high levels of chlorine, dichlor, and trichlor are ideal for leaving heavy algae blooms. 

Important note: you need to wait 24 hours before using a chlorinated pool. That’s because chlorinated shocks have pH levels that are similar to household cleaners. However, if non-chlorinated shock was used, you only have to wait 15 minutes until using the pool again. Non-chlorinated shocks usually have a pH level of 9 (which is somewhere between the pH levels of seawater and hand soap), which means it won’t irritate your skin. 

How Often Should Shock Be Applied? 

Shock needs to be applied when combined chlorine levels in your pool increase to more than 0.3 ppm. However, as long as your pool is shocked at least once a week, chlorine levels shouldn’t increase beyond this point. Keep in mind the frequency of pool maintenance might vary depending on several factors, such as how often the pool is used, the amount of rainfall, and outdoor temperatures. 

In addition to weekly pool maintenance, shock should be applied after: 

  • Long periods of hot weather – High pool temperatures can speed up bacteria growth and lower the strength of the chlorine present in the pool. 
  • Heavy rain – The contaminants that were washed away by rainwater might have increased your pool’s pH levels. 
  • At the start of every new season – It’s good practice to have your pool cleaned at the start of every season.

How to Tell if a Pool Shock Is Needed 

It’s also important to keep an eye out for the warning signs. These are: 

  • Cloudy water – Water turns murky if contaminants have polluted it. 
  • Bubbles or foam – Contaminated water has strong water surface tension, which allows bubbles and foam to form above the water surface. 
  • Strong chlorine smell – A strong chlorine smell doesn’t mean there’s too much chlorine in the pool; it means there isn’t enough chlorine to oxidize and destroy contaminants in the water. 
  • An imbalance in the pool water’s chemistry – Your pool’s pH, alkalinity, calcium, and free chlorine levels all need to be optimal. Otherwise, the pool water might become cloudy. 
  • Discolored water – Green water, along with brown or yellowish discoloration, is a sign of algae growth. 

Important note: don’t forget that routine pool inspections are also an important part of the maintenance routine. While some pool issues are obvious, others – like a slow leak or a crumbling deck – aren’t immediately noticeable. The best way to make sure repair or replacement costs don’t balloon is to have a pool company inspect your pool from time to time. Remember: the sooner these issues are detected and pool repair is conducted, the better. 

How to Keep Algae Growth Under Control 

In addition to weekly pool shocking, here are tips on keeping algae growth under control: 

  • Remove leaves and other debris from your pool – Algae feed on phosphates from dust, leaves, and debris, which is why they need to be removed from your pool. 
  • Clean the skimmer and pump strainer baskets – This will help keep water and chlorine flowing. It’s also important to have a pool repair and maintenance company inspect your pool’s filters and circulation system to make sure they’re still working properly. 
  • Brush the walls, floors, and steps – Algae can burrow their roots into cracks to implant themselves onto your pool’s surface, making them hard to remove. That’s why you need to routinely scrub off algae off of your pool’s surfaces. But if wear and tear is starting to take its toll on your pool’s walls, you might want to consider pool resurfacing
  • Make sure the pool’s chlorine content is at optimal levels – After your pool is shocked, the automatic or floating chlorinator needs to keep pumping enough chlorine to keep algae from coming back too quickly. 

Pool and Patio Pros offers a wide range of professional pool resurfacing and maintenance services.

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