Water Chemistry Basics
There are five main chemical levels that every pool owner needs to keep track of to keep their pool levels balanced.
FC – Free Chlorine – A sanitizer which keeps your pool water safe and free of bacteria and algae growth. Chlorine must be constantly replenished. (the level you need to maintain depends on your current CYA levels)
PH – Acidity/Basicity – Needs to be kept in balance to prevent irritation and protect the pool equipment. This also helps the chlorine work more efficiently and can contribute to other factors such as scale development. (7.1 to 8.0)
TA – Total Alkalinity – Appropriate levels help keep the pH in balance. High levels can cause pH to rise. As this acts as a buffer to the pH level its paramount to keep this level correct to keep the pH from fluctuating. This level can also contribute to other factors such as scale development (70 to 100ppm, sometimes higher)
CH – Calcium Hardness – Appropriate levels help prevent plaster & fiberglass damage. High levels can cause calcium scaling especially when paired with other water chemical factors. The hardness level needed changes for a liner pool as they don’t require it as a plaster and fiberglass does and having a salt water generator should be taken into consideration as a higher calcium level will cause the salt cell to develop scale on its cell more rapidly. (250 to 650ppm, vinyl lower)
CYA – Cyanuric Acid – Protects chlorine from sunlight and determines the required FC level. (outdoors 30 to 60ppm, SWG 60 to 90ppm, indoors/ always covered with auto cover 0 to 20ppm)
Here are four other chemical levels that come up frequently enough that you should at least know what they are:
CC – Combined Chlorine – CC over 0.5ppm indicates a problem. This is when chlorine has combined with other varying chemicals, most often ammonia, and causes the free to sanitize chlorine level to be lower than the total amount present in the water.
Salt – Required with a SWG (Salt Water chlorine Generator), otherwise an optional enhancement. Does make water feel less treated with chemicals and should never smell or taste of salt at correct levels. Not very effective at dealing with an active algae growth.
Borate – An optional enhancement. Hard to initially put in but has natural algaecide properties, limits the amount of chlorine needed to very small amounts, as well as keeps the pH buffered better allowing for less adjustments. There is products that allow you to bypass the tedious process to put it in by adding borates directly instead of trying to achieve the chemical reaction.
Phosphate – Doesn’t matter, despite pool store claims otherwise. It is considered any material that breaks down in the pool that can be used as a fuel for algae for bacterial growth. However, there is different methodologies that do prescribe to dealing with phosphates and that chemistry doesn’t point to them being a necessary balancing factor if the main levels are kept in check.
All nine are described in more detail below.
FC- Free Chlorine
Maintaining an appropriate FC level is the most important part of keeping your water in balance, clean, and clear. It is important that you do not allow FC level to fall too low, or you run the risk of an algae growth. If FC levels fall to zero, or you have active algae growth, the pool is probably not safe to swim in as it could be harboring bacteria as well as the algae.
Free chlorine shows the level of disinfectant available (active plus reserve) to keep your pool sanitary. FC should be tested, and chlorine added daily. If you have an automatic feeder or SWG, you can test it every couple of days. FC is consumed by UV from sunlight, and by sanitizing organic material in your pool. The level of FC you need to maintain depends on your CYA level and how much you use the pool. See the Chlorine / CYA Chart for guidelines on the appropriate FC level to maintain based on your CYA level.
Recommended ways to raise FC include: common bleach, liquid shock/chlorine, and salt water cells. Tabs in an auto feeder are acceptable but must be used with caution as over or rapid use can lead to high CYA levels as tabs contain CYA in them.
CC- Combined Chlorine
Combined chlorine is an intermediate breakdown product created in the process of sanitizing the pool. CC causes the “chlorine” smell many people associate with chlorine pools. The best example of this smell is the common place smell of it at most hotel and commercial pools as they have heavy use and deal with this frequently. If CC is above 0.5ppm, you should SLAM (Shock Level and Maintain) your pool. CC indicates that there is something in the water that the FC is in the process of breaking down. 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.
PH – Acidity/Basicity
PH indicates how acidic or basic the water is. PH should be tested daily at first. Once you gain experience with your pool, less frequent monitoring may be needed, depending on your pool’s typical rate of PH change. A PH level of 7.4 to 7.6 is ideal, but really anything between 7.2 and 8.0 is doing fine.
PH levels below 7.2 tend to make eyes sting or burn. PH below 6.8 can cause damage to metal parts, particularly pool heaters with copper heat exchange coils. High PH can lead to calcium scaling.
Many pools will drift up towards higher PH over time. This is particularly true for fresh plaster (particularly in the first month and continuing for perhaps a year) or when TA is high and the water is being aerated (because of a spa, waterfall, fountain, SWG, rain, kids splashing in the pool, etc).
For lowering PH use either muriatic acid (preferred) or dry acid. To raise PH use borax or soda ash.
TA – Total Alkalinity
Total alkalinity indicates the water’s ability to buffer PH changes. Buffering means you need to use a larger quantity of a chemical to change the PH. At low TA levels, the PH tends to swing around wildly. At high TA levels, the PH tends to drift up.
You can raise TA with baking soda. It is often best to make large TA adjustments in a couple of steps, testing the water after each one, as adding large quantities of baking soda can raise the PH a little and you don’t want the PH going out of range. However, setting the TA level first before the pH level is important as you will be chasing your tail if you don’t. If you need to lower your TA level, see How To Lower Total Alkalinity.
CH – Calcium Hardness
Calcium hardness indicates the amount of calcium in the water. Over time, water with low calcium levels will tend to dissolve calcium out of plaster, pebble, tile, stone, concrete, and to some extent fiberglass surfaces. You can prevent this from happening by keeping the water saturated with calcium. In a vinyl liner pool there is no need for calcium so a regular level of 100-150 from normal tap water is fine, though high levels can still cause problems. A plaster pool or Fiberglass pool should attempt to keep their Calcium between 250 and 650 parts per million. If you have a spa you might want to keep CH at at least 100 to 150 to reduce foaming.
You increase CH with calcium chloride, sold as a deicer and by pool stores, or calcium chloride dihydrate, sold by pools stores for increasing calcium. You lower calcium by replacing water or using a reverse osmosis water treatment.
CYA – Cyanuric Acid
Cyanuric acid, often called stabilizer or conditioner, both 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 need to use to get the same effect. It is important to know your CYA level so you can figure out what FC level to aim for. If you don’t have a SWG, CYA is typically kept between 30 and 60ppm. If you have a SWG, CYA is typically kept between 60 and 90ppm. The reason SWG are held at a high level is that while they do produce chlorine at a steady rate as long as the pump is running it is a small amount and the higher CYA allows the pool to retain the level better.
CYA can be raised with cyanuric acid. Cyanuric acid is sold under a variety of names, including Stabilizer, Conditioner, Instant Pool Water Conditioner, Stabilizer 100, Stabilizer & Conditioner, etc. Instant Pool Water Conditioner is a liquid product which is significantly more expensive than the other forms but, in my humble opinion, is worth the additional cost as it dissolves much faster which means it is not clogging up the filter long and you get the effectiveness of the CYA much faster. This is especially helpful when dealing with a green pool when you need CYA fast to help retain as much chlorine to save money on high chlorine dosage costs. I also find the application process much simpler as you just pour down skimmer, fill jug with pool water and shake to loosen any product left in jug, pour that out and repeat until it runs clear.
Solid/granular cyanuric acid (CYA) should be placed in a sock and the sock put in the skimmer basket or suspended in front of a pool return. After adding CYA you should leave the pump running for 24 hours and not backwash/clean the filter for a week. Squeezing the sock periodically will help it to dissolve faster. Test and dose chemicals in your pool assuming the amount of CYA added is in the pool according to Pool Math. CYA can be tested the day after it is fully dissolved from the sock. I highly recommend against the powder as it takes extremely long to dissolve and can cause other issues.
In nearly all cases the best way to lower CYA is to replace water. If replacement water is extremely expensive you might want to look into a reverse osmosis water treatment.
Salt
Salt is required with a SWG. Salt can also be added to the water to enhance the subjective feel of the water. For a SWG, check the manual for the correct salt level for your unit. This level will typically around 3,000, but different models vary. For improved water feel without a SWG, try levels around 2,000ppm. These levels are less then one tenth of the salt level in ocean water, which has around 35,000 ppm of salt. People vary in their ability to taste low levels of salt. A few people can taste salt levels as low as 1,000ppm, others not until 3,500ppm or more.
Salt can be added using solar salt, sold for use in water softeners (sodium chloride). You want the kind that is 99.4% pure or better and which doesn’t have any rust inhibitor or other additives. Crystals are fine. Pellets will work but I advise against them as they dissolve slightly more slowly and that can cause surface damage. Pool store salt generally costs more and is more finely ground.
Borate
Borates are an optional enhancement that helps control PH drift and provides various subjective water quality/feel improvements. If you are not intentionally using borates there is no need to test for them. When using borates, the recommended level is between 30 and 50 ppm.
Phosphate
Phosphates are sometimes removed from the pool as a way of keeping algae in check. Since chlorine is required anyway, and chlorine alone can keep algae in check (even at very high phosphate levels), and since phosphate remover can be rather expensive and annoying to use, it doesn’t make much sense to use phosphate remover except in rare and unusual situations.