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Pool shock refers to a granular oxidizer, a powdered form of chlorine that is used for pool water treatment. In addition to the noun pool shock, it can also be used as a verb, in the act of shocking a pool, or adding granular oxidizer (pool shock) to the water in sufficient quantity for the desired change.
Why do you need to shock a pool? Pools are shocked to remove combined chlorine molecules, aka chloramines from the water. Pools are also shocked to remove excess bather waste and bacteria after heavy pool use, contamination events, or for the removal of visible algae in the water. Bromine treated pools and spas also use pool shock to reactivate bromide ions in the water.
There are really 3 main reasons to shock a pool or spa, raising the Free Chlorine level high enough (10-30 ppm) to oxidize or destroy the offending contaminants. These could be called the A-B-C’s of Pool Shock.
Algae: Green, yellow, pink or black, the best algaecide is chlorine, and lots of it. Pool algae growth can be controlled with algaecide, but to kill algae and clear the pool, we use pool shock. Be sure that the pH is adjusted to 7.1-7.3 range, so the chlorine shock is most potent. Depending on the severity of the algae, 10-30 ppm is needed to kill active algae blooms. Chlorine Accelerators, such as Yellow Out work to boost your chlorine level to fight severe algae blooms, of all colors.
Bacteria & Bather Waste: Bacteria can enter the pool from many sources, most of it harmless variety, however pathogenic bacteria may also exist. Use chlorine shock to remove bacteria after heavy pool use, heavy storms, long winters or for swimmer ‘accidents’. Bather waste includes skin, hair, lotions, cosmetics and soaps, as well as sweat, urine, feces and fungus.
Chloramines, Contaminants, Cloudy Water: Combined chlorine molecules are responsible for ‘red-eye’ and a strong chlorine smell. When Chloramine levels exceed 0.5 ppm (TC-FC=CC), add enough chlorine or non-chlorine shock to break apart the combined chlorine, usually 10-20x the tested CC level.
Shocking the pool also removes organic contaminants from dirt, leaves and air pollution. There is a lot of harmless matter that gets into a pool and gets in the way of water balance and sanitation. Cloudy water can often be corrected by superchlorinating the pool, or by adding non-chlorine oxidizer, to destroy the colloidal particles that cause cloudy water.
Start-Up and Close-Down: For pools that are winterized, opening the pool is the first time of the year that pool shock is used, after the water chemistry is balanced. This helps oxidize particles, kill bacteria and algae and restore water clarity. Prior to closing the pool, pool shock is used to disinfect the water, in preparation for the long winter ahead.
Heavy Rain: Rain is pure water, distilled by evaporation, but as it falls through the air, it picks up airborne particles that wash into your pool. Air pollution, dust, pollen, algae spores can discolor the water, consume your chlorine, and affect water chemistry. During heavy storms, overhanging trees, overflowing planters or lawn areas next to the pool can wash in soil laden with bacteria and phosphates, in addition to of tree and plant debris.
Chloramines: Free chlorine becomes combined chlorine when bonded to nitrogen or ammonia. The bond renders the chlorine molecule useless and causes the pool water to smell strongly of chlorine and irritate swimmers eyes. DPD test kits can test for both Free and Total chlorine, with any difference in the test results being Combined chlorine. Shock the pool to remove chloramines when levels exceed 0.3 ppm.
How often do you need to shock a pool? Every pool is different, and pools don’t need to be shocked, unless they need to be shocked – to remove bacteria, algae, chloramines or other contaminants, or to help clear cloudy pool water or some other water problem. You can test for chloramines and you can see algae, but bacteria and other pathogens are invisible, as are nitrates, phosphates and ammonia. For this reason, many pool owners shock the pool every 3-4 weeks, even though the water may be clear and clean, just to be sure it’s disinfected and sanitary.
For pools that have hosted a party for a dozen or more swimmers, or if a pool becomes contaminated with urine, feces or vomit, a strong shocking is in order. In cases where chlorine levels have depleted, due to hot temperatures, faulty equipment, or operator error, pool shock can be used to quickly raise chlorine levels.
Another use is when a pool cannot obtain a good chlorine reading. If the water is balanced, algae-free, and does not have excessive levels of cyanuric acid, but still cannot get a good chlorine reading despite continuous additions of chlorine, there may be nitrate or ammonia levels consuming chlorine. In such cases, where you have tried everything else, try a 'triple-shock' of 3 lbs of shock, per 10,000 gallons of pool water, to destroy these invisible consumers of chlorine.
For a pool that uses chlorine or bromine for daily chlorination, there a variety of EPA approved chemicals that can be used for shocking the pool. The best one for your pool may depend on your pool type, or if you have issues with hard water or high cyanuric acid levels.
Calcium Hypochlorite: Cal Hypo for short, is the most economical pool shock you can buy. It is available in 65% and 73% strength, pH level of 12, and is not stabilized.
Sodium Dichlor: Dichlor for short, is stabilized pool shock, with cyanuric acid as protection from the sun. 56% strength, with a nearly neutral pH level. Adds no calcium to the pool.
Potassium Monopersulfate: Non-Chlorine Shock for short, oxidizes pool water in a chlorine free formula that is not affected by sunlight, leaves no residue, and adds only oxygen.
Which pool shock to use? Most pools can use Cal Hypo, but for those in hard water areas, with concerns of growing calcium hardness levels, Sodium Dichlor or Non-Chlorine shock may be a better choice. Each pound of Cal Hypo will add 5-7 ppm to calcium hardness levels. Vinyl pools benefit from non-chlorine shock, which won’t bleach or fade vinyl liners, and also don’t cloud the water or leave behind a dusty residue. Pools with growing cyanuric acid levels may want to avoid Dichlor shock; each pound adds a small amount of cyanuric acid.
In The Swim Cal Hypo pool shock is packaged in easy-opening 1 lb. bags, in a 65% Pool Shock or 73% Super Shock. We also have Instant Pool Shock in 1 lb bottles, and Cal-Chlor, in 25 and 50 lb buckets; easier to use when adding large amounts of pool shock at one time. Chlorine-Free, Cal Hypo and Dichlor pool shocks are all available in 1 lb. bags.
A new type of pool shock is now available, in the form of a single use gel cap that you place in your skimmer (don’t add other types of pool shock to your skimmer). Safe-N-Clean Pool Oxidizer is a non-chlorine oxygen based oxidizer that treats up to 30,000 gallons.
Waiting to swim after shocking. Follow package instructions, which will guide you in how long to wait after shocking before swimming. Heavy shocking with granular chlorine will generally require 24-48 hours before the chlorine level has dropped to safe swimming levels (below 5 ppm). Lithium and Non-Chlorine shock labels typically allow immediate swimming, but check the package label, to be sure.
Chlorine-Free shock treatments contain oxygen, with salts of potassium, and it does have many advantages when compared to chlorine-based pool shock, and it costs the same. Strengths vary among brands, but most non-chlorine shock is 38% - 42% Potassium Monopersulfate, aka MPS.
Chlorine shock treatments come in two types - Cal Hypo and Dichlor. Calcium Hypochlorite is the most common, strongest and cheapest pool shock, available in two strengths, Shock and Super Shock. Dichlor is a stabilized granular pool shock, made with stabilizer to protect it from the sun and keep it active longer during the day.
How much pool shock to use? You’ll need to first know the amount of water in your pool, give or take a few hundred gallons. If you aren’t sure, take some measurements and consult an online pool volume calculator. Generally speaking, the dosage amount of pool shock is 1 lb. per 10,000 gallons, but consult the shock package label. Depending on the severity of the problem you are addressing, you may need a double or triple dose, to successfully fix the issue.
For chloramine removal, shock the pool to reach a Free Chlorine level that is 10 to 20 times the amount of measured chloramines. For algae removal, 30 ppm is a generally accepted target, but you may use more or less, depending on the severity of the algae bloom. You can check poolcalculator for amounts of pool shock needed to reach a specific ppm level. Another rule of thumb in algae removal is that if the water is still green, you need more. For boosting chlorine levels that have dropped to zero, 1 pound of pool shock will usually be sufficient.
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. Visibility (or lack thereof) is another way to gauge severity of algae bloom. If you can see the shallow end floor, use a double-dose of shock. But if you can only see down for 12" or 24" into the water, use a 3x-6x treatment dosage.
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.
How long does pool shock last? Granular chlorine products will lose only a small percentage of potency, when stored in a cool, dry and dark location. When stored in a shed or garage however, the varying temperature and humidity levels will begin to solidify the contents, and within a few years, the 1 lb. plastic bags will deteriorate.
For longer and safer storage, we recommend buying loose Cal Hypo in buckets, or Non-Chlorine shock. Store in a dark and cool location, with a very tight lid to keep out moisture and contamination and to prevent off-gassing.
For vinyl liner pools, undissolved granules resting directly on vinyl can bleach, fade or corrode soft vinyl surfaces. Pre-dissolving is accomplished by filling a clean 5-gal bucket full of pool water.
Pour in 1 or 2 lbs. of granular pool shock directly into the water (always add chemicals to water, not water to chemicals). Stir with a yardstick or suitable paddle for several minutes to dissolve the granules.
Pour the solution around the edge of the pool, and as the bucket is almost empty stop, add more water to dissolve remaining granules in the bottom of the bucket. Vinyl pools can avoid pre-dissolving by using non-chlorine shock or lithium hypo shock.
When and Why to Shock a Pool
Other Uses for Pool Shock
How to Shock a Pool
Storing Pool Shock: Keep in a cool, dry area, separated from other pool chemicals, and out of reach of children. Shock is more safely stored if removed from the cardboard box and placed inside a clean bucket or storage bin with a tight fitting lid. Do not store half-used bags of shock, which could spill, become contaminated, or take on moisture.
Using Pool Shock: Cut bag carefully with scissors, and pour into the water while walking along the pool edge. Use a pool brush to distribute, and sweep or wash any spills into the pool. Vinyl liner pools should pre-dissolve granular shock, unless using quick dissolving Oxy Shock.
Never Mix Pool Shock: With anything except Water. Pool shock is very reactive and when mixed with any substance other than water, it can release toxic gases, ignite or explode. Never place shock into a chlorinator or floater, or add to the skimmer, always add directly to the pool.