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What’s in Your Water

Maintaining a safe swimming pool is really pretty easy, and there are five chemical levels that every pool owner needs to pay attention to:

* FC – Free Chlorine – This is a sanitizer which keeps your pool water safe. Chlorine must be constantly replenished, and the how much you need depends on your CYA level.
* pH – Acidity/Alkalinity – Your pH needs to be kept in balance to prevent irritation (eyes) and to protect your pool equipment. You need to maintain your pH between 7.2 to 7.8 ppm.
* TA – Total Alkalinity – proper TA levels help keep the pH in balance. High levels can cause pH to rise. We recommend TA to be between 80-120 ppm.
* CH – Calcium Hardness – Appropriate levels (200-400 ppm in plaster pools-vinyl can be lower) help prevent plaster damage. High levels can cause calcium scaling.
* CYA – Cyanuric Acid – CYA protects chlorine from sunlight and determines the required FC level. We recommend 30 to 50 for outdoor pools, and 0 to 20 for indoor pools.

There are three other items that come up frequently enough that you should at least know what they are:

* Salt – Required with a SWCG to produce chlorine. Salt can also be added to your pool to improve “feel”t.
* Borate – Borates are an optional additive that can help to buffer your pH.
* Phosphates – These do not matter, despite what the pool stores say! They know you can’t, or won’t test for them, so they get to sell you stuff to make them go away!

To learn a bit more about these, all eight are described in more detail below.

FC – Free Chlorine

Free chlorine is the measure of the level of disinfecting chlorine available in your pool, active and in reserve, to keep your pool sanitary. For a pool to hold a consistent FC level, FC should be tested and chlorine added daily. If you are lucky enough to have an automatic feeder or SWCG, you can probably test it every couple of days. FC is consumed by sunlight and by breaking down organic material in your pool, be it algae, sweat or urea (if you don’t know, don’t ask!). The amount of FC you need to maintain depends on your CYA level as well as by how much you use the pool. For help, see the Chlorine / CYA Chart for guidelines on the appropriate FC level to maintain based upon your CYA level.

It is very important that you do not allow your FC to get too low, as you run the risk of getting algae. If you allow your FC to get down to zero, or if you have algae, then the pool is not safe to swim in. Continually maintaining an appropriate FC level is the most important part of keeping your pool water in balance.

Bleach, liquid chlorine, liquid shock, tri-chlor tablets/pucks/sticks, di-chlor powder, cal-hypo powder/capsules, and lithium hypochlorite will all raise the FC level. It is important to note that if you use bleach, you will need to get the type without any additives or special features. These are typically labeled “unscented” or “original scent”. In addition to chlorine, tri-chlor and di-chlor will also add CYA, and lower the pH of the water. Cal-hypo adds chlorine and calcium, and lithium hypochlorite tends to be quite expensive. See “How to Chlorinate Your Pool” for help in choosing what kind of chlorine to use.

When adding chlorine, it is most efficient to raise the FC level in the evening, since none will be lost to sunlight until the next morning. Chlorine normally goes down by itself, but if you must lower the FC level quickly, you can use a chlorine neutralizer (sodium thiosulfate or sodium meta bisulfite).

CC – Combined Chlorine

Combined chlorine is an intermediate breakdown product that is created in the process of sanitizing the pool. CC causes the chlorine “smell” that many people associate with chlorine pools. If CC is above 0.5, you will need to shock your pool. CC indicates that there is something in the water that the FC is in the process of breaking down or attacking. Typically, in an outdoor pool, CC will normally stay at or near zero as long as you maintain an appropriate FC level and the pool gets some direct sunlight.

Potassium monopersulfate (which is a common non-chlorine shock product) will often show up on tests as CC. There is a special reagent you can get to neutralize the potassium monopersulfate so you can get a true CC reading, if you are using potassium monopersulfate to shock your pool.

TC – Total Chlorine

Total chlorine is the sum total of FC plus CC. Inexpensive chlorine tests, such as the common OTO test, which shows the TC level as different shades of yellow, measures TC. This is because it is easier to test for than FC and CC. In normal operation, TC can be used as if it was FC, because the CC level is usually zero. Remember that you cannot use the OTO test, or any other tests that only measure TC, when you have algae or certain other problems. This is because anything that might cause the CC level to be above zero, such as algae, makes the TC level different from the FC level. In these situations, TC is useless on its own and does not help you properly care for your pool.

pH – Acidity/Alkalinity

pH is a measurement that indicates how acidic or basic the water is. pH should be tested daily at initially. Once you are more familiar with your pool, less frequent monitoring may be appropriate, depending on your pool’s rate of pH change. pH levels between 7.5 and 7.8 are considered ideal, while levels between 7.2 and 7.8 are acceptable for swimming.

pH levels below 7.2 tend to make eyes sting or burn,and can cause damage to plaster. pH below 6.8 can cause damage to metal parts, most commonly pool heaters with copper heat exchange coils. High pH, on the other hand, can lead to calcium scaling. pH contributes to the CSI, which indicates the tendency for plaster damage or calcium scaling. Aeration will tend to cause the pH to rise, and can be addressed by lowering TA.

Many pools have a tendency to drift up towards higher pH over time. This is particularly true for freshly plastered pools (particularly in the first month and continuing for up to a year), or when TA is high and the water is being aerated (aeration can be caused by a spa, waterfall, fountain, SWCG, rain, kids splashing in the pool, etc).

If you need to raise pH, it can be done with borax or soda ash/washing soda. Soda ash/washing soda will increase TA more than borax will. To lower pH, you can use muriatic acid or dry acid. How much you will need for a given pH change depends on several other numbers, most importantly your TA, and borate levels, if you are using borates. Higher TA and/or borate levels will cause you to need larger amounts of chemicals to adjust pH.

TA – Total Alkalinity

Total alkalinity is an indication of the water’s ability to buffer pH changes. Buffering means that you will need to use a larger quantity of a chemical to change the pH. At low TA levels, the pH will tend to swing around wildly. At higher TA levels, the pH wants to drift upward. TA also contributes to the CSI, which indicates the tendency for plaster damage or calcium scaling.

The ideal TA level depends on several factors. If you are using acidic chlorine sources, such as tri-chlor or di-chlor, you will want to keep TA on the high side, perhaps between 100 and 120. If you have a SWCG, or if you have water features such as a spa, waterfall, or fountain, you will want to keep TA on the low side, between 60 and 80. Otherwise, levels between 70 and 90 are fine. Pools with plaster surfaces should always factor their CSI into the preferred TA level decision. Vinyl liner pools can tolerate high TA levels reasonably well.

You can raise TA with baking soda (sodium bicarbonate). It is often best to make large TA adjustments in a couple of steps, testing your water after each step. Adding baking soda will affect the pH, and you don’t want the pH going out of range. If you find yourself needing to lower your TA level, see How To Lower Total Alkalinity.

ATA or CTA – Adjusted or Corrected Total Alkalinity

An adjustment is sometimes made to the measured TA, subtracting out the cyanurate alkalinity. This allows for it to more closely approximate the alkalinity as CaCO3 (calcium carbonate). This number is only used when calculating LSI, and you should always use the TA result directly from the test.

CH – Calcium Hardness

Calcium hardness is the measure of how much calcium in your water. Over time, water that has low calcium levels will tend to dissolve calcium out of plaster, pebble, tile, stone, concrete, and to some extent even fiberglass surfaces. You can prevent this from happening by keeping the water saturated with the proper levels of calcium. In a vinyl lined pool there is no need for calcium (check your equipment specs, however, as equipment manufacturers will often require a calcium level for warranty purposes), but high calcium levels can still cause problems. A plaster pool should have CH levels between 200 and 400 (per the National Plasterers Council, or NPC). Calcium helps fiberglass pools resist staining and cobalt spotting. If you have a spa you might want to keep CH at at least 100 to 150, which helps to reduce foaming. CH is a contributor to the CSI, which indicates the tendency for plaster damage or calcium scaling.

You can increase CH with calcium chloride, sold as a de-icer, or calcium chloride dihydrate, sold by pools stores for increasing calcium. You can lower calcium by replacing water or using a reverse osmosis water treatment.

TH – Total Hardness

Total hardness is the amount of calcium hardness and magnesium hardness combined in your pool water. Most test strips will report TH instead of CH. The ratio of calcium to magnesium varies, and for an approximation you can multiply TH by two thirds to get a rough estimate of CH. You can also test for CH with a good quality test kit, such as the Taylor K2006-C.

CYA – Cyanuric Acid

Cyanuric acid, also called stabilizer or conditioner, protects FC from sunlight and lowers the effective strength of the FC, by holding some of the FC in reserve. The higher your CYA level, the more FC you will need to have to get the same effect. It is important to know your CYA level so you know what FC level to maintain. If you don’t have a SWCG, or problems from large amounts of sunlight, CYA is typically kept between 30 and 50. If you have a SWCG, or have high levels of direct sunlight, CYA is typically kept between 70 and 80. If you are using an ORP controller, you will want to keep CYA below 50.

You increase your CYA by adding cyanuric acid, often sold as stabilizer or conditioner. CYA can be purchased as a solid or as a liquid. The liquid costs a lot more, and is typically viewed as not worth the extra expense. Solid stabilizer (CYA) can take up to a week to fully register when testing, so don’t check your CYA level for a week after adding. For this reason, we suggest adding less than you think you might need initially. You can always add CYA, but removing excess can be costly. Solid stabilizer is best added by placing it in a sock in the skimmer basket, and then allowing the pump to run for 24 hours after adding solid stabilizer. You should avoid back-washing or cleaning the filter for a week after adding CYA.

The most common way to lower CYA is to replace water. If replacement water is extremely expensive, or you live in areas with high CH in the tap water or where drought is common, you might want to look into a reverse osmosis treatment, which retains a large portion of your water and provides better quality water.


Salt is required to make chkorine with a SWCG. Salt can also be added to the water if you wish to alter the subjective feel of the water. For a SWCG, check the manual for the correct salt level for your unit, as these tend to vary. This level, depending on manufacturer, will typically be around 2,900-3,200. For improved water feel without a SWCG, try adding salt to around 2,000-3,000. These levels are less then one tenth of the salt level in ocean water, which has a salt content of about 35,000 ppm. People vary in their ability to taste low levels of salt; a few people can taste salt levels as low as 1,000 ppm, while others cannot taste it until 3,500 ppm or more.

Having salt in the water just slightly increases the risk of corrosion, particularly to surfaces that water splashes onto and can evaporate. This water can leave high concentrations of salt behind. This is normally only a problem for stone work that comes in contact with the water and is made from softer kinds of stone (Flagstone, for example). There is a lack of solid information about the salt corrosion risks, and the level at which this occurs, for many materials, leading to debate about the overall level of risk. Most people with salt in their water do not experience problems, but it is something to consider when looking into SWCG’s for your pool.

Salt can be added using solar salt, often sold for use in water softeners (sodium chloride). You want to make sure that you get the kind that is 99.4% pure or better, and does not have any rust inhibitors or other additives. Crystals are fine, and pellets will work but dissolve slightly more slowly, and should be brushed around to help dissolve. Pool store salt generally costs more and is more finely ground, but pellets dissolve quickly enough and will work fine. It is typically easier to find the crystals as opposed to the pellets. Potassium chloride can also be used, but you will need 17% more, and it costs more.


Borates are used as an optional enhancement that helps control pH drift, helps keep algae in check, and provides various subjective water quality/feel improvements. If you are not intentionally using borates, then there is no need to test for them. When you are using borates, the recommended level is between 30 and 50 ppm.

With borate levels at 30 or above, there is a chance that pets that drink large quantities of water from the pool may have some problems. Since pets should always be trained to not drink pool water, this really should not be a problem.

You increase borates by adding borax and acid, or by the addition of boric acid. 20 Mule Team Borax can be found in the laundry detergent section of most large grocery stores, and boric acid is available over the Internet. You lower borates by replacing water or by a reverse osmosis treatment. Some products that contain borates are Proteam’s Supreme, Bioguard’s Optimizer Plus, Omni’s Maximizer, and Poolife’s Endure.


Phosphates are sometimes removed from the pool as a way of keeping algae in check. Since chlorine is required in a swimming pool, and chlorine alone can keep algae in check, even at very high phosphate levels, it doesn’t make much sense to use phosphate remover except in rare and unusual situations. A properly sanitized chlorine pool will not have algae, and even pools with algae can be addressed with high doses of chlorine. Phosphate removers are sold by pool stores whenever you tell them you have algae! They know that you cannot test for them, so it is a relatively easy sell for them. Phosphate removers are rather expensive and annoying to use, and are generally not needed.