"Shocking" your pool sounds like a fix. Most of the time, it isn't.
Shocking is a specific chemical event: raising free chlorine to a high enough level, for long enough, that it kills off whatever's been hiding in your water. Bacteria, algae spores, organic contamination from a heavy rainstorm, sunscreen and sweat after a pool party. That's what it's for.
It is not a routine maintenance step. It is not something you do "every week" because the bag says so. And shocking the wrong way — at the wrong dose, with the wrong product, for the wrong reason — can make your problem worse, not better.
When to actually shock
You should shock your pool when:
You see early algae. Walls feel slick. Water has a green or yellow tinge. You can spot dots on the floor that didn't wipe off.
Combined chlorine (CC) is above 0.5 ppm. That means you have spent chlorine accumulating and your sanitizer isn't keeping up.
After a heavy rainstorm. Texas thunderstorms dump organics, debris, and sometimes runoff into your pool. Test, then shock if FC dropped to zero.
After a big pool party. Sunscreen, sweat, body oils, and the rest of it all consume chlorine.
After someone gets sick in the pool. Different protocol — a CDC-style remediation, not a regular shock. Call us if this happens.
You should not shock your pool when:
It's a Tuesday and the bag of shock has been sitting in your shed.
Your free chlorine is already 5+ ppm.
Your pool just looks "a little cloudy" but FC and CC are normal. Cloudiness usually isn't a chlorine problem.
How much shock to add
Here's where most homeowners get it wrong.
The right shock dose isn't "one bag per 10,000 gallons." It's a calculation that depends on your current CYA level. In a pool with 30 ppm CYA, you need to push FC up to about 12 ppm. In a pool with 50 ppm CYA, you need to push FC up to about 20 ppm. In a pool with 80 ppm CYA — which is unfortunately common — you need to push FC up to about 32 ppm. (This is the "Chlorine/CYA shock chart" if you want to look it up.)
If you shock at the standard label dose without knowing your CYA, you usually under-shock. The water looks like it cleared, but algae spores survived because the FC level never actually got high enough for long enough. Two days later you're greener than before.
The Pool Shock Calculator handles this math for you. You give it your gallons, your current FC, your CYA, and what shock product you have. It tells you the dose that will actually work.
Picking the right shock product
Cal-hypo (calcium hypochlorite) — most common. Works fast. But it adds calcium, which over time will drive your calcium hardness up. Not great if your CH is already 350+.
Liquid chlorine (sodium hypochlorite) — what most pool service companies actually use. Doesn't add calcium or stabilizer. Acts immediately. The most predictable shock product for repeat use.
Dichlor and trichlor "shock" products — be careful. These add CYA every time you use them. If you shock with dichlor 3–4 times a summer, you're slowly poisoning your stabilizer level.
Non-chlorine shock (potassium monopersulfate) — useful for clearing combined chlorine, less useful for actually killing algae. Has its place, but it's not a substitute for a real shock when you need one.
If you don't know what you have on the shelf, look at the active ingredient on the back. That tells you everything you need to know.
How to actually shock
Test the water before you start. FC, CC, pH, CYA. If pH is above 7.6, lower it first — chlorine is more effective at lower pH, and you'll waste shock at high pH.
Calculate the dose with the calculator. Don't eyeball it.
Pre-dissolve cal-hypo in a bucket of water before adding (never pour granular cal-hypo straight into a pool — you'll bleach the surface).
Add at dusk or after dark. UV from the sun destroys chlorine fast, especially the high concentrations during a shock. Shocking at noon wastes half the dose.
Run the pump for 8+ hours. Continuous circulation is what gets the chlorine to every corner.
Retest the next morning. If FC dropped below your target shock level, repeat. The shock isn't done until FC has stayed above the threshold and started coming down on its own.
When shocking isn't going to work
If your pool is seriously green — chartreuse, can't see the bottom — shocking once probably won't fix it. You're looking at a multi-day process or a "drain and clean." We do both. A green-to-clean visit usually pays for itself in saved chemicals and saved weeks.
Run the Pool Shock Calculator or call Thomas directly at (214) 399-7347.
