Every pool owner needs to monitor five important chemical levels:

  • FC – Free Chlorine– This sanitizer keeps your pool water safe and bacteria-free. Chlorine must be regularly replenished.
  • PH – Acidity/Basicity– This must be balanced to prevent irritation and protect pool equipment. (7.2-7.8)
  • TA – Total Alkalinity– Correct levels balance the pH. High levels can cause pH to rise. (60-20ppm, sometimes higher)
  • CH – Calcium Hardness– Correct levels help prevent plaster damage. High levels can cause calcium scaling. (220-350ppm, lower for vinyl)
  • CYA – Cyanuric Acid– Protects chlorine from sunlight and determines the required FC level. (outdoors 30-50ppm, SWG 70-80ppm, indoors 0-20ppm)

There are four other chemical levels you should also know about:

  • CC – Combined Chlorine– CC over 0.5ppm indicates a problem.
  • Salt– Required with a SWG (saltwater chlorine generator), otherwise an option.
  • Borate– Optional enhancement.
  • Phosphate– Not relevant, despite claims by pool stores.

These nine chemicals are further described below:

FC – Free Chlorine 

Balanced pool water depends on the correct FC level. Allowing FC to get too low incurs the risk of algae. If FC reaches zero, or there’s algae, the pool is unsafe to swim in.

Free chlorine shows the level of disinfecting chlorine available (active plus reserve) to keep the pool sanitized. Test FC and add chlorine daily. You can test every couple of days if you have an automatic feeder or SWG. Sunlight consumes FC, and FC is also consumed by breaking down organic material in the pool. The correct level of FC to maintain depends on CYA levels and how the pool is used. Follow chlorine/CYA guidelines to maintain the appropriate FC level based on your CYA level.

To raise FC, use household bleach, liquid chlorine, or saltwater chlorine generators (SWG).

CC – Combined Chlorine 

Combined chlorine is an intermediate breakdown product created during the sanitization process of the pool. CC causes the familiar “chlorine” smell typically associated with chlorine pools. If CC is above 0.5ppm, you should treat your pool because CC indicates a component in the water the FC is breaking down. In outdoor pools, CC normally remains at or near zero as long as you maintain appropriate FC levels and the pool gets some direct sunlight.

Potassium monopersulfate (a popular non-chlorine shock) reads on FAS-DPD chlorine tests as CC. You can get a special reagent to neutralize the potassium monopersulfate to get an accurate CC reading.

PH – Acidity/Basicity

PH measures how basic or acidic the pool water is. Test PH daily at first. Once you’re more experienced, you can monitor less frequently depending on the pool’s typical rate of PH change. The ideal PH level is 7.7-7.8 is ideal, but anything between 7.2-7.8 is acceptable, while levels between 7.2-8.0 are fine for swimming.

PH levels below 7.2 can cause eye irritation. PH below 6.8 can damage metal components, particularly pool heaters with copper heat exchange coils. High PH can cause calcium scaling.

PH levels in many pools tend to drift higher over time. This particularly applies to fresh plaster (especially during the first month and continuing for up to a year) or when TA is high and the water is being aerated due to a spa, waterfall, fountain, SWG, rain, swimmers splashing in the pool, etc.

To lower 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.

Use baking soda to raise TA. It’s better to make large TA adjustments in gradual steps and test the water afterward. Adding large quantities of baking soda at a time can raise PH, and you want to avoid levels moving out of range.

CH – Calcium Hardness

Calcium hardness is the amount of calcium in the water. Left unchecked, pool water with low calcium levels can dissolve the calcium from plaster, pebble, tile, stone, concrete, and to some extent, fiberglass surfaces. Prevent this by keeping the water saturated with calcium. There’s no need for calcium in a vinyl liner pool, but high levels can still cause problems. Plaster pools without a SWG should maintain CH levels between 250-350. With a SWG, maintain CH at between 350-450. Calcium helps prevent staining and cobalt spotting in fiberglass pools. In spas, CH levels should be maintained at 100-150 to reduce foaming.

Use calcium chloride to increase CH. This is sold as a deicer and by pool stores, or use calcium chloride dihydrate sold in pools stores to increase calcium. Lower calcium by replacing the water or using a reverse osmosis water treatment.

CYA – Cyanuric Acid 

Cyanuric acid, popularly called stabilizer or conditioner, protects FC from sunlight and lowers the effective strength of the FC by holding some it aside as reserve. The higher the CYA level, the more FC is needed to get the same effect. You must know your CYA level to determine the appropriate FC level. If you don’t have a SWG, CYA is typically maintained between 30-50ppm. If you have a SWG, CYA is typically maintained between 70-80ppm.

You can raise CYA with cyanuric acid. Cyanuric acid can be called Conditioner, Stabilizer, Instant Pool Water Conditioner, Stabilizer 100, Stabilizer & Conditioner, among others. Instant Pool Water Conditioner is a liquid  that is considerably more expensive than other types.

Put solid/granular cyanuric acid (CYA) into a sock, and then placed into the skimmer basket or positioned in front of a pool return. After adding CYA, leave the pump running for 24 hours and don’t backwash/clean the filter for a week. Periodically squeeze the sock periodically to help it dissolve faster. Test and add chemicals to the pool, but ensure the amount of CYA added is correct according to an online calculator. Test CYA the day after it has fully dissolved from the sock.

In nearly all cases, the best way to lower CYA is to replace the water. If replacement water is extremely expensive, you might want to look into a reverse osmosis water treatment.

Salt

Saltwater chlorine generators (SWG), require salt. It can also be added to the water to enhance the feel. Check the manual of your SWG to determine the correct salt level for the model. Levels are typically around 3,000, but different models vary. To improve the feel of the water without a SWG, keep levels around 2,000ppm. These levels are less then one-tenth of the salt level contained in ocean water, which has around 35,000 ppm of salt. The ability to taste low levels of salt varies among people. A few can taste salt levels as low as 1,000ppm, others not until 3,500ppm or higher.

You can add salt using solar salt. This is used in water softeners (sodium chloride). Choose a 99.4% pure or higher that has no rust inhibitors or other additives. Crystals are better as pellets dissolve a little more slowly. Pool store salt is more finely ground and can be more expensive, but pellets dissolve quickly enough so the extra cost is unnecessary.

Borate

Borate is an optional enhancement that manages PH drift and provides some water quality/feel improvements. If you don’t use borates, it’s unnecessary to test for them. If you choose to use them, recommended levels are between 30-50 ppm.

Phosphate

Removing phosphates from the pool is a means of controlling algae. Since chlorine is required, which controls algae even at very high phosphate levels, and phosphate remover can be expensive and bothersome to use, this chemical really only needs to be used in unusual or occasional situations.