DIY · water chemistry 101

Water chemistry, without the jargon.

Six things you have to watch, what each does, and the ranges that keep a pool clear.

Pool water chemistry basics

Video coming soon

Video walkthrough coming soon.

The short version

A pool is a chemistry experiment you swim in. There are six numbers to track. Get them in range and the water stays clear. Let one drift too far and the others start fighting you — low pH eats plaster, high pH clouds the water, low CYA burns off your chlorine in an afternoon, high alkalinity locks the pH.

Nobody memorizes this on day one. Bookmark this page. Re-read it when a number looks weird.

The six numbers

In order of how often they move. Chlorine first (daily), then pH (2–3x a week), then the rest (monthly-ish).

01
Free chlorine (FC) — ideal 2–4 ppm
The active sanitizer. This is what kills bacteria and algae. Below 1 ppm and the pool tips green in a day. Above 5 ppm and swimmers complain about the smell. If your CYA is high (see below) you need proportionally more FC — the two are linked.
02
Combined chlorine (CC) — ideal 0 ppm
Chlorine that’s already reacted with contaminants (sweat, sunscreen, urine). The famous “pool smell” is actually CC, not chlorine itself. If CC is above 0.5 ppm you need to shock. FC minus CC = free chlorine actually available.
03
pH — ideal 7.4–7.6
How acidic or basic the water is. Below 7.2 and it starts eating plaster and equipment gaskets; above 7.8 and chlorine effectiveness plummets and water turns cloudy. pH climbs slowly on its own (from aeration), falls when you add chlorine. Muriatic acid lowers it; soda ash raises it.
04
Total alkalinity (TA) — ideal 80–120 ppm
The pH buffer. Alkalinity is what keeps pH from bouncing every time you sneeze near the pool. Low TA and pH will crash on you daily; high TA and pH gets locked at 8+ and won’t come down. Adjust alkalinity first, pH second — always.
05
Cyanuric acid (CYA) — ideal 30–50 ppm
Sunscreen for your chlorine. Without CYA, sunlight destroys FC in under 2 hours. With CYA in range, FC holds all day. But too much CYA (above ~70 ppm) starts “locking” the chlorine — the FC number reads fine but nothing’s actually sanitizing. This is the #1 cause of “my chlorine is high but my pool is green.”
06
Calcium hardness (CH) — ideal 200–400 ppm
How much calcium is dissolved in the water. Below 150 and the water starts pulling calcium out of your plaster (etching, roughness). Above 500 and you get scale on the tile line and inside the heater. Move slow — CH goes up easy, down only by draining and refilling.
07
Salt (for saltwater pools) — ideal 2,700–3,400 ppm
Only relevant if you have a salt chlorine generator. Your salt cell splits NaCl into chlorine. Too low and the cell throws a low-salt error and stops producing. Too high and it starts eroding the cell plates faster.

How to actually test

Three tiers, in order of accuracy:

Test strips — fine for a quick FC + pH check between real tests. Good for daily eyeballing; unreliable for CYA or high-CC pools.
Drop-count kit (Taylor K-2006, ~$70) — the DIY gold standard. Covers all six numbers above. Same reagents pool stores use. Reagents last ~6 months before drift.
Photon reader (~$700) — what I use. Digital, no color-matching. Overkill for a single pool owner, but if you’re on Chemical Only I test with this weekly.

Whichever tier you use, test at the same time of day and away from the return jets (where the fresh chemistry hasn’t mixed yet). Elbow-deep sample from the deep end is the standard.

Common mistakes I see

Chasing pH before alkalinity. If TA is low, pH won’t hold no matter how much you add. Fix TA first, then pH follows.
Ignoring CYA. If your CYA is over 80, your only fix is to drain and refill some water. There is no chemical that lowers CYA.
Mixing chlorine and acid. Never in the same bucket, never within an hour of each other in the pool. Chlorine gas is not a joke.
Shocking without brushing. Algae hides in the plaster micro-cracks. If you don’t brush before/during a shock, you leave it alive.
Adding stabilizer “just in case.” Stabilizer is CYA. Adding it when you don’t need it is the #1 reason pools go green mid-summer.

Chemical only

Chemistry, handled.

You just clean.

Weekly photon-reader testing and all chemicals included — flat $150/mo. You keep doing the physical cleaning.

Doing it yourself

With Chemical Only

Chemicals

$110–$140 / mo at Lowes

Included — tabs, shock, acid, stabilizer, algaecide, whatever your pool needs

Water testing

Strip test (drifts every 6 mo) or monthly supply-store dip

Photon reader — same weekly, on-site

Your time

30–60 min / week

None — you just clean

Monthly total

$110–$140 + your time

$150 flat

Text me about Chemical Only