Free chlorineFC
The active sanitizer — the chlorine that's still on the clock.
What it is. Free chlorine is the chlorine that hasn't been used up yet and is still available to kill germs, bacteria, and algae. It's the single most important number for keeping your water safe to swim in.
Why it matters. If free chlorine drops to zero, your pool stops being protected and algae or bacteria can take hold within a day or two. Keeping a steady, modest level is what keeps the water clear and safe — you don't need a lot, you need it to never run out.
If it's too lowBelow your target, sanitizing slows down. Let it hit zero and you're one warm day away from a green pool. This is the reading you never want to ignore.
If it's too highVery high chlorine can irritate eyes and skin and fade swimsuits, but it's rarely dangerous and it falls on its own. We'd rather see you a little high than at zero.
Total & combined chlorineTC / CC
Total minus free tells you if your chlorine is getting tired.
What it is. Total chlorine is all the chlorine in the water — the free (still-working) chlorine plus the 'combined' chlorine that has already reacted with sweat, sunscreen, and other contaminants. Subtract free from total and what's left is combined chlorine, also called chloramines.
Why it matters. Combined chlorine is the stuff that makes a pool smell strongly of 'chlorine' and stings swimmers' eyes. Ironically, that smell means you have too little effective chlorine, not too much. A high combined number is your cue that the water needs a shock to burn off the spent chlorine.
If it's too lowLow combined chlorine is exactly what you want — it means your free chlorine is doing its job cleanly.
If it's too highWhen combined chlorine climbs past about 0.5 ppm, the water can smell harsh and irritate eyes. The fix is to shock the pool, which clears out the spent chlorine and lets fresh free chlorine take over.
pH
How acidic or basic your water is — the comfort dial.
What it is. pH measures how acidic or basic the water is on a scale from 0 to 14, where 7 is neutral. Pool water sits slightly basic. It's the reading that most affects how the water feels and how well everything else works.
Why it matters. pH touches everything: it controls how comfortable the water feels on skin and eyes, how effective your chlorine is, and whether the water protects or attacks your pool's surfaces and equipment. Get pH right and most other problems get easier.
If it's too lowLow pH (acidic) water stings eyes, can corrode metal fittings and heater parts, and slowly etches plaster. It also makes chlorine burn off faster.
If it's too highHigh pH (basic) water makes chlorine sluggish, can cause cloudy water and scale buildup, and leaves a skin feeling some describe as 'slippery.' It's a common cause of 'my chlorine is fine but the water still looks off.'
Total alkalinityTA
pH's bodyguard — it keeps pH from bouncing around.
What it is. Total alkalinity is a measure of how much 'buffer' is in the water — the dissolved compounds that absorb acids and bases before they can swing your pH. Think of it as a shock absorber for pH.
Why it matters. When alkalinity is in range, your pH stays put and is easy to adjust. When it's too low, pH becomes unstable and 'bounces' all over the place with every rain or chemical addition. Setting alkalinity first is why we usually fix it before chasing pH.
If it's too lowLow alkalinity means pH won't hold still — you'll correct it and it drifts right back. Water can also turn aggressive toward surfaces and metal.
If it's too highHigh alkalinity makes pH stubborn and prone to drifting upward, and can lead to cloudy water and scale. You'll find yourself adding acid more often.
Cyanuric acidCYA
Your chlorine's sunscreen.
What it is. Cyanuric acid, also called stabilizer or conditioner, is a compound that shields chlorine from being destroyed by sunlight. Outdoors, unprotected chlorine can be gone in a couple of hours; CYA makes it last.
Why it matters. CYA is the hidden number behind your chlorine. The right amount of CYA lets a modest chlorine level last all day. Too much, and your chlorine becomes lazy — present on the test but weak at actually sanitizing. That's why a good app reads your chlorine target against your CYA, not in isolation.
If it's too lowWith too little CYA, sunlight burns off your chlorine fast and you struggle to hold a level through a sunny afternoon.
If it's too highToo much CYA 'locks up' chlorine — you need a higher and higher chlorine level to get the same sanitizing power. The only real fix is to dilute by replacing some water, so it's worth not overshooting in the first place.
Calcium hardnessCH
How much dissolved calcium your water carries.
What it is. Calcium hardness measures the amount of dissolved calcium in the water. Water naturally wants a certain amount of calcium in it — give it too little and it goes looking for some.
Why it matters. Calcium is about protecting your investment. Water that's starved of calcium will pull it out of plaster, grout, and concrete, slowly damaging the surface. Too much and you get chalky scale. The right level keeps the water 'satisfied' and your surfaces intact.
If it's too lowLow calcium makes water aggressive — it etches plaster and can corrode surfaces as it tries to dissolve calcium back into itself. Fiberglass and vinyl pools tolerate low calcium far better than plaster.
If it's too highHigh calcium can leave cloudy water and white scale deposits on tile and equipment, especially when pH or alkalinity is also high.
Salt
Only matters if you run a saltwater system.
What it is. Salt level is how much dissolved salt is in the water. In a saltwater pool, a salt-chlorine generator passes that salty water across a metal cell that turns the salt into chlorine on the spot — so 'saltwater' pools are still chlorine pools, they just make their own.
Why it matters. If you have a salt system, the generator needs salt in a specific window to produce chlorine. Too little and it can't keep up; too much and it can damage the cell. If you have a traditional chlorine pool, you can ignore this number entirely.
If it's too lowLow salt and the generator throttles back or shuts off, so chlorine production drops and your free chlorine can sag. You'll add pool salt to bring it back up.
If it's too highToo much salt can corrode the cell and other components, and the only fix is to dilute with fresh water. Add salt gradually so you never overshoot.
Water temperature
The quiet factor behind how fast everything happens.
What it is. Water temperature isn't something you dose for, but it changes how the chemistry behaves. Warm water speeds up reactions, helps algae grow, and burns through chlorine faster; cold water slows everything down.
Why it matters. Temperature is why summer pools need more chlorine and more attention than the same pool in spring. A good chemistry engine factors temperature into scale and corrosion math, so the advice you get in July isn't the same as in October — and that's a feature, not an inconsistency.
If it's too lowCold water holds chlorine longer and grows algae slowly, so demand drops — you'll add less.
If it's too highWarm water accelerates chlorine loss and algae growth, so demand rises. Hot summer stretches are when staying on top of testing pays off most.
