Total chlorine is the sum of free chlorine and combined chlorine. The level of total chlorine will always be higher than or equal to the level of free chlorine.
Combined Chlorine (CC):
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. CC indicates that there is something in the water that the FC is in the process of breaking down.
Free Chlorine (FC):
Maintaining an appropriate FC level is the most important part of keeping your water in balance. It is important that you do not allow FC to get too low, or you run the risk of getting algae. If FC gets down to zero, or you have algae, the pool is not safe to swim in. Always maintain a residual reading of AT LEAST 1ppm.
Free chlorine shows the level of disinfecting chlorine available (active plus reserve) to keep your pool sanitary. FC should be tested, and chlorine added daily. If you have an automatic feeder or saltwater generator, you can test it every couple of days. FC is consumed by sunlight, and by breaking down organic material in your pool. The level of FC you need to maintain depends on your cyanuric acid (stabilizer) level and how much you use the pool.
PH indicates how acidic or basic the water is. A PH level of 7.0 and 7.6 is ideal. The ability of chlorine to disinfect at this level is also optimal. 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, cause burning eyes/nose, dry/itchy scalp, dull/cloudy water.
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 Total Alkalinity is high.
Total Alkalinity (TA):
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.
Stabilizer (Cynauric acid/CYA):
Like a sunscreen for your chlorine. Cyanuric acid combines with chlorine to protect it from the UV rays of the sun, but releases it on demand when it is needed to sanitize the water.
Cyanuric acid, often called stabilizer Free Chlorine 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 or problems from extremely high amounts of sunlight, CYA is typically kept between 30 and 50. If you have a SWG or very high levels of direct sunlight, CYA is typically kept between 70 and 80.
You increase CYA by adding cyanuric acid, often sold as stabilizer. CYA is available as a solid and as a liquid. The liquid costs a lot more, and generally isn’t worth the extra expense. Solid stabilizer can take up to a week to fully register on the test, so don’t retest your CYA level for a week after adding some. Solid stabilizer is best added by placing it in a sock in the skimmer basket. The pump should be run for 24 hours after adding solid stabilizer and you should avoid backwashing/cleaning the filter for a week.
In nearly all cases the best way to lower CYA is to replace water – a partial drain or longer backwash times may be recommended.
Calcium Hardness (CH):
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, though high levels can still cause problems. A plaster pool should have CH levels between 250 and 350 if possible. 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 to reduce foaming.
You increase CH with calcium chloride, by the addition calcium chloride dihydrate, sold by pools stores for increasing calcium. You lower calcium by utilizing longer backwash times, a partial drain, then replacing water.