Pool Shock Treatments
Pool shock is used as a noun, as in the granular oxidizer, and a verb, as in to shock the pool. Shocking the pool is the process of introducing an oxidant to the pool water in such a quantity, that electrons are taken (reduced) from contaminants; destroying chloramines, pathogens, algae, and organic matter less than a micron in size. In other words, shocking the pool destroys all foreign matter, to purify and disinfect the pool water.
When and Why to Shock a Pool
- To reduce the build-up of micro-contaminants and organic matter
- To destroy both harmless and pathogenic bacteria in the water
- To remove combined chlorine molecules, aka chloramines
- To kill algae blooms, or to treat cloudy pool water
Other Uses for Pool Shock
- Organic stain removal on a plaster pool surface
- Raising chlorine level fast when discovered near zero
- Raising or Keeping chlorine level during pump, filter or salt system troubles
- Spot treating small algae colonies in corners and crevices
How to Shock a Pool
- Check pH level and adjust if needed to a low-range pH of 7.1-7.3
- Clean the pool to remove leaves and debris, remove pool cleaner
- Determine how much shock is needed to fix the situation
- Add shock directly to the pool, vinyl pools should pre-dissolve chlorine shock
- Add shock when the sun is not shining directly on the pool
- Run filter and brush pool to help distribute and circulate the shock
How Often Should you Shock a Pool?
For most pools without water problems, a monthly shocking would be advised to help clear the water of unwanted solids and contaminants, listed as #1 reason to shock a pool, above. These invisible solids can build-up and make more work for your filter and sanitizer. Non-Chlorine shock is best for general contaminant removal.
For reason #2 above, to destroy bacteria as needed, if the chlorine level dropped close to zero, or after severe storms, or if you had very heavy and active pool use, or if there was a contamination of fecal matter, dead animals or other such problems. This could take 1-3 lbs per 10,000 gals of pool water, to reach 10-30 ppm for several hours, depending on the type of bacteria suspected. Super Shock can deliver 10 ppm per 10,000 gals, per 1 lb. bag, in good water conditions – you may need more. Chlorine shock is best for killing bacteria.
For reason #3 above, to remove combined chlorine from the water. Chloramines are what makes a pool smell strongly of chlorine, and turns swimmer’s eyes red. If your Total Chlorine test is more than 0.3 ppm higher than your Free Chlorine test, shocking to a level 10x-20x higher than the measured difference, is recommended for removal of the smelly chloramines (TC-FC=CC). Non-Chlorine shock is best for removing chloramines.
For reason #4 above, to kill algae blooms, pool shock is a great algaecide, as we like to say, when used correctly. When shocking to remove algae, be sure the pH is 7.2-ish and keep adding shock until the water turns a blue-grey color, with no hint of green left, then add another pound for good measure. Depending on the extent of the algae bloom, this could require 3-6 lbs per 10,000 gals of pool water. 30 ppm, for several hours, is the target level for killing algae. Chlorine shock is best for algae blooms.
How Much Shock should I Use?
That depends on what problem you are trying to solve, and the current condition of the water. If the water looks good and you just want to boost the chlorine level a bit, add ½ bag per 10,000 gallons. If the water is hazy or cloudy use a full bag per 10K gallons, and if there is algae use 2-6 lbs per 10,000 gals, depending on the extent of the algae bloom. A chlorine level of between 10 ppm and 30 ppm, sustained for several hours, is usually needed to remove algae, bacteria and chloramines. Super Shock can deliver 10 ppm per 10,000 gals, per 1 lb. bag, in good water conditions – you may need more.
In many cases, the dosage listed on a bag of shock will be effective on blue and clear water, but if you have very cloudy or very green water, a 3x – 6x treatment dosage is not unusual. The higher the level of solids, algae or chloramines that are in the water, the more pool shock will be needed to oxidize the matter.
Remember that a Low pH level is crucial to successfully shocking a pool. At a pH level of 8.0, over half of your shock is ineffective and wasted. At a pH level of 7.2 however, over 90% of your shock will become active algae and bacteria killers.
Missing the mark sometimes happens when trying to clear adverse water conditions. If you still have a strong chlorine level 12 hours after shocking, and the water appearance is improving with filtering - mission accomplished (probably). But, if the chlorine level is zero again after 12 hours and pool doesn’t look much better, you may have missed the mark, or threshold off breakpoint chlorination. Try again.
Which Pool Shock is Best?
Like the previous question, the best type of shock may also depend on what problem you are trying to solve, or prevent. Or maybe it also depends on your definition of “best”, what are you looking for?
Strongest Pool Shock: That would be our Super Pool Shock, with 73% available chlorine. Regular cal hypo has 65%, and dichlor has 56% available chlorine. The best shock for algae and bacteria treatment, and for organic stain removal on plaster pools.
Stabilized Pool Shock: Often used on pools at startup, to add some cyanuric acid to the pool, or used on outdoor fountains that are drained frequently. Also useful for hand-feeding chlorine during equipment failure, DiChlor shock lasts longer during sunny days.
Swim Immediately Shock: Both the Chlorine Free Shock and Lithium Hypo Shock are fine like play sand and dissolve almost instantly. Chlorine shocks require a 12-24 hour waiting period for swimming.
No Residue Shock: Again the Chlorine Free and Lithium Hypo shocks are the best shock for not leaving behind messy ‘shock dust’, residue from the binders used in Cal Hypo and DiChlor pool shocks.
For Vinyl Liners: All shocks can be used in vinyl pools, but chlorine shock should be pre-dissolved so that granules don’t lay on the vinyl pool floor. Chlorine Free and Lithium pool shocks won’t fade vinyl.
Nothing Added: A drawback of Cal Hypo for hard water areas is that it adds to your calcium hardness level. A drawback for some Dichlor shock users is that it adds to your cyanuric acid level. Chlorine Free or Lithium shock can be used in pools with concerns of rising calcium or cyanuric acid levels.
For Algae & Bacteria: For complete destruction of algae blooms and bacteria contamination, use the strongest chlorine pool shock available, Super Pool Shock, and add enough to get the job done. 10-30 ppm is suggested, or 1-6 lbs per 10,000 gals, depending on the level of contamination.
For Contaminants & Chloramines: You could use chlorine shock for general pool water oxidation, or chloramine removal – but the gentler Chlorine Free shock does the same thing; why not try some?
Chlorine Shock Vs. Non-Chlorine Shock – Pros & Cons
Chlorine-Free shock treatments contain oxygen, with salts of potassium, and it does have many advantages when compared to the 3 types of chlorine-based pool shock. Strengths vary among brands, but most non-chlorine shock is 38% - 42% Potassium Monopersulfate, aka MPS.
Non-Chlorine Shock Pros:
- Quick-release, fast acting, no residue, no odor
- pH balanced at near neutral pH
- Oxidizes organic contaminants to purify water
- Removes chloramines and ammonia
- No Pre-Dissolving needed, pour into pool
- Swim Immediately, no waiting period
- Won't bleach or fade vinyl liners or swim suits
- Won't add calcium or cyanuric acid to pool
Non-Chlorine Shock Cons:
- Costs about 10% more than chlorine shock
- Not very effective for algae treatment
- Not very effective for bacteria treatment
Chlorine shock treatments come in 3 types - Cal Hypo, Lithium Hypo and Dichlor. Calcium Hypochlorite is the most common, strongest and cheapest pool shock, available in two strengths, Shock and Super Shock. Lithium hypochlorite contains no calcium and dissolves instantly, but is less powerful (and more costly) than Cal Hypo. Dichlor is a stabilized granular pool shock, made with stabilizer to protect it from the sun and keep it active longer during the day.
Chlorine Shock Pros:
- Quick-release, fast acting
- Oxidizes organic contaminants to purify water
- Removes chloramines and ammonia
- Kills algae and bacteria easily to disinfect water
- Cheapest way to shock the pool (Cal Hypo)
Chlorine Shock Cons:
- Pre-dissolving granular is recommended for surface protection
- Waiting period of 12-24 hours is generally recommended
- High levels of chlorine can be harsh on soft and shiny surfaces
- Cal Hypo adds calcium, and Dichlor adds cyanuric acid ('Pro' in some cases)
- More hazardous to store and use, and has a strong odor
- Residue of 'shock dust' is often left, or water becomes cloudy
Related Blog Posts