The active sanitizer — the chlorine actually available to kill bacteria, algae, and break down sweat and sunscreen. This is the single most important reading for safe water.

• Sunlight burns it off fast. UV destroys unprotected chlorine in hours — a sunny afternoon can drop FC by half. This is why CYA (stabilizer) matters.
• Swimmers and heat consume it. A pool party, hot weather, or organic debris (leaves, pollen) all eat chlorine quickly. Test more often in summer.
• Heavy rain dilutes it and washes in contaminants, often dropping FC and requiring a top-up afterward.
• Too low → unsafe. Below 1 ppm, bacteria and algae take hold within a day. Below the FC/CYA minimum, you risk an algae bloom.
• Too high → irritation. Above 5 ppm can irritate eyes and skin; wait for it to drop before swimming.

How acidic or basic the water is. pH controls how effective your chlorine is, how comfortable the water feels, and whether it corrodes or scales your equipment.

• Chlorine works best at 7.2–7.6. At pH 8.0, more than half your chlorine is inactive — you can have "enough" FC on paper but weak sanitizing in practice.
• pH naturally rises over time. Aeration (waterfalls, fountains, splashing, even a windy day) off-gasses CO₂ and pushes pH up. Most pools drift upward and need acid periodically.
• Fresh plaster/concrete pools leach lime for the first year, driving pH and alkalinity up persistently.
• Trichlor and dichlor chlorine products are acidic and slowly drag pH down.
• Too low → corrosive, stings eyes, etches plaster, eats metal. Too high → cloudy water, scale, and weak chlorine.

A buffer that keeps pH stable. Think of TA as pH's shock absorber — the right amount stops pH from bouncing around with every chemical addition or rainfall.

• Low TA → pH bounce. With too little buffer, pH swings wildly and unpredictably; a small acid addition can crash it. Frustrating to keep stable.
• High TA → pH creep. Too much buffer makes pH stubbornly drift upward and resist correction, often causing cloudy water and scaling.
• Acidic rain and acid additions lower TA over time; aeration raises pH without changing TA, so the two are adjusted together.
• Adjust TA before pH — getting the buffer right first makes pH far easier to dial in and hold.

How much dissolved calcium is in the water. Water "wants" a certain amount of calcium — too little and it pulls calcium out of your pool surfaces; too much and it deposits scale.

• Too low → the water gets hungry. Soft water leaches calcium from plaster, grout, and concrete, etching surfaces and shortening their life. Especially damaging in plaster pools.
• Too high → scale. Excess calcium deposits as white crust on tile, heaters, and salt cells, and clouds the water.
• Fill water varies hugely. Hard-water regions add calcium with every top-up; soft or rainwater fills stay low. Know your tap water.
• Evaporation concentrates it. Water leaves, calcium stays — so CH slowly climbs over a hot season. The only practical way to lower it is to drain and refill.

Sunscreen for your chlorine. CYA shields free chlorine from UV breakdown so it lasts through the day instead of burning off in a few hours.

• Outdoor pools need it. With zero CYA in direct sun, chlorine vanishes almost as fast as you add it. 30–50 ppm dramatically extends its life.
• Indoor pools and spas don't. No sun means no UV loss — CYA just gets in the way, so it's left out entirely.
• Higher CYA needs higher FC. The more stabilizer, the more chlorine you must carry to stay effective (the FC/CYA ratio). Too much CYA "locks" chlorine and invites algae.
• It only goes up. CYA doesn't break down on its own and trichlor/dichlor products keep adding it. The only way down is dilution — drain and refill.
• Rain and splash-out are the only things that lower it naturally, and slowly.

A single number combining pH, alkalinity, calcium, and temperature that tells you whether water is corrosive, balanced, or scaling. It's the overall health summary of your water's chemistry.

• Negative → corrosive. The water is hungry and will dissolve plaster, grout, and metal to satisfy its calcium demand.
• Positive → scaling. The water is saturated and drops calcium carbonate as crusty scale on surfaces and equipment.
• Temperature matters. Warm water scales more readily — a heated spa at 100°F behaves very differently from a cool pool at 60°F with identical chemistry.
• Balance, not perfection. You can hit ideal FC and pH and still have corrosive or scaling water if calcium and alkalinity are off. LSI catches what individual readings miss.