Question 1

Can I mix chlorine and shock?

Accepted Answer

Never mix them in concentrated form — that combination produces chlorine gas or can ignite. Add each chemical directly to pool water separately, with the pump running, and wait at least 15-30 minutes between additions. Shock and chlorine (granular or liquid) are both oxidizers; the hazard is when they react with each other before they're diluted.

Question 2

What order do I add chemicals?

Accepted Answer

Adjust pH first (it affects how every other chemical performs), then shock or adjust chlorine, then address total alkalinity, then calcium hardness, and finally CYA if needed. Add one chemical at a time, wait a full pump cycle, then retest before adding the next. Never combine chemicals in the same bucket or dosing cup — even trace amounts of two products reacting can be dangerous.

Question 3

Why does my pH keep rising?

Accepted Answer

Most pools trend pH upward naturally. The main drivers are: CO2 off-gassing from aeration (water features, jets, splashing), high total alkalinity locking the buffer high, and some sanitizers (especially trichlor in a feeder) being acidic — while liquid chlorine (sodium hypochlorite) is alkaline and pushes pH up with every dose. A pool with high TA and liquid chlorine will drift pH high persistently; the fix is to bring TA into the 80-120 ppm range, which gives pH room to settle.

Question 4

Do I really need CYA?

Accepted Answer

For an outdoor chlorine or salt pool, yes — without CYA, sunlight destroys free chlorine in 30-60 minutes. CYA (cyanuric acid, also called stabilizer) extends that to hours or days by forming a weak bond with free chlorine that UV light cannot break as quickly. The caveat: CYA only helps up to about 50 ppm; above 80-100 ppm it starts "locking up" chlorine so tightly that effective sanitizing drops even when your test kit shows plenty of free chlorine. For indoor pools, CYA is usually unnecessary.

Question 5

When should I shock?

Accepted Answer

Shock when combined chlorine (the difference between total chlorine and free chlorine) exceeds 0.3-0.5 ppm, after heavy bather load or a rainstorm, when the water looks dull or starts to cloud, or on a weekly or bi-weekly maintenance schedule regardless. Evening is the best time — UV light burns off unstabilized shock quickly, so shocking at night gives the product hours to work before the sun rises.

Question 6

How long after adding chemicals can I swim?

Accepted Answer

For most chemicals, wait at least 15-30 minutes with the pump running and then retest. After shocking, wait until free chlorine drops back below 5 ppm (the standard "safe to swim" threshold) before entering the water — this can take anywhere from one hour to overnight depending on the shock dose, sunlight, and circulation. With pH-adjusting acids, wait at least 30 minutes and confirm pH is back inside 7.2-7.6 before swimming.

Question 7

What is combined chlorine (and why does it matter)?

Accepted Answer

Combined chlorine is the portion of total chlorine that has already reacted with contaminants (sweat, body oils, ammonia from urine) to form chloramines. Chloramines are what cause the "pool smell" and eye irritation people associate with a heavily chlorinated pool — a pool that smells strongly of chlorine often has too little free chlorine, not too much. Your test kit reports combined chlorine as total chlorine minus free chlorine. Anything above 0.3-0.5 ppm is a signal to shock.

Question 8

Are algaecides necessary?

Accepted Answer

Not as a routine chemical if your free chlorine and CYA levels are consistently in range — adequate free chlorine at the correct CYA ratio already suppresses algae. Algaecides are most useful as a supplemental treatment after a green-pool recovery (to catch stragglers once free chlorine is restored) or in pools where maintaining consistent chlorine is difficult (heavy shade, frequent rain dilution, intermittent service). For most maintained pools, keeping pH, free chlorine, and CYA in range does the same job without the added chemical cost.

Eight pool chemistry questions, answered.

Run your readings through the app.

Eight pool chemistry questions, answered.

01Can I mix chlorine and shock?

02What order do I add chemicals?

03Why does my pH keep rising?

04Do I really need CYA?

05When should I shock?

06How long after adding chemicals can I swim?

07What is combined chlorine (and why does it matter)?

08Are algaecides necessary?

Run your readings through the app.