Pool chemistry is a system, not a list. Most new pool techs memorize the 6 standard tests, hit the target ranges, and assume they understand the chemistry. Then a pool turns cloudy in week 2 and they cannot figure out why because they were treating the numbers as independent dials instead of a coupled system. This guide is the operator reference for the 6 chemistry inputs that actually matter, the relationships between them, and the common operator mistakes that cause callbacks. Written for techs in their first 12 months and operators training new hires.
TL;DR
- The 6 numbers: free chlorine, pH, total alkalinity, calcium hardness, cyanuric acid (CYA), and TDS
- Free chlorine target depends on CYA. The standard 1 to 3 ppm rule is wrong without context
- pH and total alkalinity are coupled. Adjusting one without considering the other causes drift
- CYA is the most ignored number in pool service and the cause of most "mystery" callbacks
- LSI (Langelier Saturation Index) is the integration of all 6 numbers into one verdict on water balance
- Test kit: Taylor K-2006 is industry standard. Test strips are a backup, not a primary tool
Free chlorine: the only number customers see
Free chlorine (FC) is the active disinfectant in the water. It is also the only chemistry number most homeowners can recognize, which is why a low FC reading is the most common reason for a complaint call. Standard residential target is 1 to 3 ppm for a pool with CYA around 30 to 50.
The mistake new techs make is hitting "1 to 3 ppm" regardless of CYA. The correct target scales with CYA. At CYA 30, the FC target is 1.5 to 3 ppm. At CYA 60, it is 4 to 7 ppm. At CYA 80, it is 5 to 9 ppm. Trouble Free Pool publishes the full chart and it is the reference every service tech should bookmark.
pH: the lever that affects everything
pH measures acidity on a 0 to 14 scale. Pool target is 7.4 to 7.6, slightly above neutral. pH affects how effective chlorine is, whether scale forms on equipment, and whether the water etches plaster. Drift up over time on most pools because of off-gassing CO2; salt pools drift up faster.
Adjustment: muriatic acid drops pH (and total alkalinity). Soda ash raises pH. Most operators add 16 to 32 oz of acid per visit on a typical residential pool to hold pH in the target window. Customers who think "the pool is fine, no need to acid" are the customers whose plaster surface is being slowly etched.
Total alkalinity: pH's buffer
Total alkalinity (TA) is the water's resistance to pH change. Target 60 to 120 ppm depending on water source. Below 60, pH bounces all over the place. Above 120, pH is harder to lower and scale formation accelerates.
TA and pH are coupled. Acid lowers both. Soda ash raises both. To raise pH without raising TA, use borax. To lower TA without dropping pH dramatically, add small acid doses with vigorous aeration. New techs often raise TA with bicarb when they should raise pH instead, which causes the bounce they were trying to fix.
Calcium hardness: the slow killer
Calcium hardness (CH) is the dissolved calcium in the water. Target 200 to 400 ppm for plaster pools. Below 200, the water leaches calcium from the plaster surface, which over years pits and degrades the finish. Above 400 (or higher in hard-water markets), scale forms on tile, heater elements, and salt cells.
Calcium is hard to remove without partial drain. Easier to keep it in target by testing monthly and adding calcium chloride when low. Phoenix, Las Vegas, and other hard-water markets routinely run 500 to 700 ppm CH from the tap, which means partial drains and refills are part of the chemistry maintenance, not a once-a-decade event.
“Calcium hardness is the number that does not bite for 6 months. Then it bites for 6 years.”
Cyanuric acid: the most ignored number
Cyanuric acid (CYA) is chlorine stabilizer. Target 30 to 50 ppm for chlorine pools, 70 to 80 for saltwater pools. Below 30, UV destroys chlorine in hours. Above 100, chlorine becomes inefficient and the only fix is dilution.
CYA is the chemistry number that causes the most "the pool turned green for no reason" callbacks. Trichlor tablets contain CYA, so every season of tablet feeding adds 30 to 60 ppm to the pool. By year 2 or 3, CYA is at 100+ and the chlorine is no longer effective regardless of how much you add. The fix is drain and refill, not more chlorine.
Test CYA monthly. Track it over time. Most customers do not understand CYA until you explain it once. After that they accept partial drains as part of pool ownership.
TDS: the diagnostic, not the target
Total Dissolved Solids (TDS) is everything dissolved in the water (salt, calcium, minerals, leftover algaecide, etc.). Target under 1,500 ppm for chlorine pools, but salt pools naturally run 3,000 to 4,500 ppm because of the salt itself. TDS is more of a diagnostic than a target: high TDS usually means the pool needs a partial drain.
Tech tip: track TDS month over month. A sudden jump is a sign something has been overdosed (algaecide, salt, etc.). A slow climb over 18 months is normal evaporation and refill cycle.
LSI: the integration of all 6 numbers
Langelier Saturation Index (LSI) is the integration of pH, temperature, calcium hardness, total alkalinity, CYA, and TDS into a single number that tells you whether the water is balanced, scaling, or corrosive. Target is -0.3 to +0.3 (some operators target tighter, -0.1 to +0.1).
Negative LSI means the water is hungry: it will pull calcium out of plaster, etch surfaces, and corrode metal components. Positive LSI means the water is depositing: scale forms on heater elements, tile, and salt cells. Most service techs never calculate LSI manually. The LSI calculator is the easier path.
Test kit: get it right or do not test
The Taylor K-2006 is the standard pool service test kit. FAS-DPD chlorine titration (accurate to 0.2 ppm above 5 ppm where strips fall apart), plus pH, TA, CH, and CYA tests. $80 to $110 plus $40 to $70 per year in reagents.
Test strips are useful as a 30-second rough check between actual tests, not as primary chemistry. A tech who relies on strips for FC chlorine is going to dose wrong on a pool with high CYA every time, because strips top out around 5 ppm and cannot read the elevated targets needed at high CYA.
Common chemistry mistakes that cause callbacks
- Adding trichlor tablets every visit, ignoring that CYA is now at 110 ppm and the pool is uncovered for it
- Hitting "1 to 3 ppm chlorine" as a hard rule without checking CYA
- Using soda ash to raise pH when alkalinity is already high. Causes drift and scale
- Skipping calcium hardness tests for 6 months in a hard-water market. Plaster surface gets etched silently
- Treating TDS as the cause of cloudy water. It is rarely the actual cause
- Using test strips for FAS-DPD chlorine titration. Inaccurate above 5 ppm where it matters most
The order of operations on a single visit
A clean chemistry visit follows the same order every time. Test FC and pH first (the two that drive immediate dosing). Test TA and CH on a monthly cadence. Test CYA at the start and end of the season. Adjust pH down with acid before adjusting FC up, because high pH reduces chlorine efficiency. Add chlorine last, in the deep end, with the pump running.
Document everything. The chemistry log is what you reference 4 months later when a customer asks "why is my pool cloudy now." Without it, you are guessing. With it, you can pull up the trend and explain the cause in 90 seconds.
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