FC - Free Chlorine
Free chlorine shows the level of disinfecting chlorine available to keep your pool sanitary. FC should be tested and chlorine added daily, unless you have an automatic feeder or SWG in which case you can test it every couple of days. FC is consumed by sunlight and when it breaks down organic material in your pool. The level of FC you need to maintain depends on your CYA level and how much your pool is used. See The Pool Calculator, Chlorine/CYA Chart by Chemgeek, or Ben's Best Guess Chart for guidelines on the appropriate FC level to maintain based on your CYA level. It is important that you do not allow FC to get too low or you run the risk of getting algae and/or having an unsafe pool.
FC is raised with bleach, trichlor tablets/pucks/sticks, dichlor powder, cal-hypo powder/capsules, or lithium hypochlorite. Only use bleach without any additives, typically labeled unscented or "original scent". Trichlor and dichlor also add CYA and lower PH. Cal-hypo also adds calcium. Lithium hypochlorite tends to be quite expensive. It is most efficient to raise the FC level in the evening since none will be lost to sunlight until the next morning. FC normally goes down by itself. If you are in a hurry you can lower FC with a chlorine neutralizer.
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 some people associate with chlorine pools. If CC is above 0.5 you should shock your pool. CC indicates that there is something in the water that the FC is in the process of breaking down. 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 (a common non-chlorine shock) 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.
TC - Total Chlorine
Total chlorine is the sum of FC plus CC. Inexpensive chlorine tests, such as the common OTO test which shows TC as different shades of yellow, normally show TC because it is easier to test for than FC and CC. In normal operation TC can be used as if it was FC because CC is usually zero. However when you have algae or some other problems, CC levels can be significant and TC becomes useless.
PH - Acidity/Alkalinity
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 appropriate depending on your pool's typical rate of PH change. PH between 7.2 and 7.8 is suitable for swimming and between 7.4 and 7.6 is ideal. PH outside of those ranges tends to make eyes sting or burn. PH below 7.0 can cause damage to metal parts, particularly pool heaters. PH contributes to the CSI, which indicates the tendency for plaster damage or calcium scaling. Chlorine is somewhat more effective at lower PH. Large amounts of aeration will cause the PH to rise. This can be mitigated by increasing the PH slightly.
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).
You can raise PH with borax or soda ash/washing soda. Soda ash/washing soda will also increase TA more than borax will. You can lower PH with muriatic acid or dry acid. How much you will need for a given PH change depends on your TA. Higher TA levels cause you to need larger amounts of chemicals to change the PH.
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 slowly, or even quickly in extreme cases. TA 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 trichlor or dichlor, keep TA on the high side, perhaps between 100 and 120. If you have a SWG, or if you commonly run water features such as a spa, waterfall, or fountain, keep TA on the low side, between 70 and 90. Pools with plaster surfaces should factor their CSI into the preferred TA level decision. Pools with vinyl liners can tolerate high TA levels reasonably well.
You can raise TA with baking soda. It is best not to add more than 2 lbs of baking soda per 10,000 gallons at one time. You can lower TA by lowering the PH to between 7.0 and 7.2 with acid and then aerating the water to bring the PH back up. Aeration can be supplied by spa jets, waterfall, fountain, rain, kids splashing, compressed air, or by aiming a return up towards the surface so it breaks the surface of the water and causes bubbles. This process is then repeated until you reach the desired TA.
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 lined pool there is no need for calcium. A plaster pool should have CH levels between 200 and 400 if possible. There is some indication that fiberglass pools require some CH. If you have a spa you might want to keep CH at at least 100 to 150 to reduce foaming. CH contributes to the CSI which indicates the tendency for paster damage or calcium scaling.
You increase CH with calcium chloride, sold as a deicer, or calcium chloride dihydrate, sold by pools stores for increasing calcium. You lower calcium by replacing water.
TH - Total Hardness
Total hardness is the sum of calcium hardness and magnesium hardness. Most test strips report TH instead of CH. The ratio of calcium to magnesium varies. As an approximation you can multiply TH by two thirds to get a rough estimate of CH.
CYA - Cyanuric Acid
Cyanuric acid, often called stabilizer or sometimes conditioner, both protects FC from sunlight and lowers the effective strength of the FC, requiring you to use more FC 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 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 60 and 80. If you are using an ORP controller, keep CYA below 50.
You increase CYA by adding stabilizer. Stabilizer can take up to a week to dissolve, so don't retest your CYA level for a week after adding some. You can add stabilizer to the skimmer, in which case you must not backwash or clean the filter for the next week. You can also put stabilizer in a sock and either put the sock in the skimmer or hang it near a return. To lower CYA you should replace water. There is a CYA reducer product sold in a few places but it is relatively expensive and I have not heard of anyone trying it.
Borates can be used to control PH rising due to a SWG or high aeration levels. Borates also help to control algae. Various subjective water quality/feel improvements are also attributed to borates. If you are not intentionally using borates there is no need to test for them. To use borates to control PH rise add borates to between 30 to 50.
With borate levels at 30 or above there is a chance that pets which regularly drink large amounts of water from the pool might have some problems. Pets should be trained to not drink pool water if you are using borates.
You increase borates by adding borax and acid. 20 Mule Team Borax can be found in the laundry detergent section of most large grocery stores. You lower borates by replacing water. Proteam's Supreme, Bioguard's Optimizer Plus, Omni's Maximizer, and Poolife's Endure all contain borates.
Salt is required with a SWG. Salt can also be added to the water to enhance the subjective feel of the water. Other than those uses, you don't need to test for or worry about the salt level. For a SWG, check the manual for the correct salt level for your unit. This level will typically around 3000, but different models vary. For improved water feel, try levels between 1000 and 2000. These levels are less then one tenth of the salt level in ocean water. People vary in their ability to taste low levels of salt. A few people can taste salt around 1000, others not until 3500 or more.
Having salt in the water slightly increases the risk of corrosion, particularly to surfaces that water splashes onto where the water can evaporate, leaving high concentrations of salt behind. This is normally only a problem for stone work above the water line made from softer kinds of stone. There is a lack of solid information about the salt corrosion risks for many materials, leading to debate about the overall level of risk. Most people with salt in their water do not experience any problems.
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 dissolve slightly more slowly. Pool store salt generally costs more and is more finely ground, but even pellets dissolve quickly enough so that isn't really an advantage. Potassium chloride can be used but you will need 17% more of it and it costs more.
CSI - Calcite Saturation Index
The calcite saturation index is a tool for estimating the likelihood of plaster corrosion or calcite scaling. The LSI, Langelier Saturation Index, is a very similar but slightly less acurate measure. The CSI uses PH, TA, CH, CYA, temperature, Borate, and Salt levels to estimate the likelihood of problems. A low saturation index means the water is likely to dissolve calcite out of paster, pebble, tile, stone, and concrete surfaces (and perhaps fiberglass) which will eventually cause damage. A high saturation index means the water is likely to deposit calcite scale on the walls of the pool and in the plumbing.
CSI is most sensitive to changes in PH. With a plaster pool, it is best to try and get your CSI near zero, so that changes in PH won't shift your pool too far towards corrosion or scaling. With a vinyl pool the CSI can be kept negative, which makes it very unlikely that PH changes could get the CSI into the range of scaling risk.