Pool chlorine low after shocking? Here's why

You dumped in a jug of shock last night, and this morning the test kit shows almost nothing. Frustrating, and very common. There are five usual reasons, and a couple of quick tests will tell you which one you're dealing with.

1. The test is wrong

Start here, because it's the cheapest fix. Standard DPD color tests can "bleach out" when chlorine is very high. The CDC says this can happen above 10 ppm, and Pool & Spa News puts it at 10 to 15 ppm. The sample goes pale or clear and looks like zero chlorine.

If you just added a big dose and the reading makes no sense, retest with an FAS-DPD kit. You count drops instead of matching colors, and Taylor says it reads up to 20 ppm. Check that your reagents or strips haven't expired, too.

2. The sun is burning it off

Sunlight destroys chlorine fast in an outdoor pool that has no stabilizer (cyanuric acid, CYA). AQUA Magazine says bright sun can break down most of the chlorine "within an hour." Sources don't agree on an exact rate, but everyone agrees it's quick.

Stabilizer acts like sunscreen for chlorine. Test your CYA. If it's near zero in an outdoor pool, that's very likely your answer, and the stabilizer calculator will give you the dose. Raising a 15,000-gallon pool from 0 to 30 ppm takes about 3 lb 12 oz of cyanuric acid.

Shocking in the evening, as HTH recommends, helps for the same reason: the chlorine gets the whole night to work before the sun is back on it.

3. Something is eating it

Chlorine gets used up as it destroys what swimmers and the weather bring in. The CDC lists pee, poop, sweat and dirt from swimmers, plus sunlight. Wojtowicz adds algae and windblown debris. If there's a lot of that in the water, a normal shock dose gets swallowed before it can build up.

Two signs point here. Combined chlorine is high (total chlorine minus free chlorine above about 0.4 ppm), or chlorine drops a lot overnight with the cover on and no sun. Both mean there's still something in the water using it up, very often algae you can't see yet.

The fix is to keep adding chlorine until it holds. Brush the walls, run the filter continuously, shock again, and repeat until free chlorine stays up from evening to morning. If you can see any green, follow the green pool guide.

4. Too much stabilizer

This one catches people who use chlorine tablets all summer. Every trichlor tablet adds stabilizer along with chlorine (about 0.6 ppm of CYA for each 1 ppm of chlorine), and stabilizer never breaks down on its own.

A little CYA protects chlorine from the sun. Too much makes it slow to kill germs and algae. CDC's pool code says CYA reduces chlorine's germ-killing rate, caps it at 90 ppm for public pools, and says free chlorine should go up as CYA rises. NSW Health and Wojtowicz both found that past about 30 to 50 ppm, extra CYA adds very little sun protection anyway.

If your CYA is high, the only fix is to replace some water. The stabilizer calculator shows how much to drain: going from 100 ppm to 50 ppm means replacing about 50% of the water.

5. Your chlorine is weak

Liquid chlorine loses strength as it sits, faster in heat. The Pool & Hot Tub Alliance gives its half-life as about 180 days at 77°F and about 48 days at 95°F. So a jug that sat in a hot garage for a couple of months can be noticeably weaker than its label says.

Buy fresh, store it somewhere cool and shaded, and if you suspect an old jug, add a measured dose and retest an hour later. If chlorine rose less than the chlorine calculator predicted, the jug is weak.

Heat makes everything faster

Warm water uses up chlorine faster. CDC's code mentions loss from "higher water temperatures," and Shasta County says chlorine goes faster once water passes about 78 to 82°F. Expect to add more during a heat wave.

Track it

Write down your free chlorine every evening and morning for a few days on the test log. The overnight drop tells you a lot. A big drop overnight points to something eating chlorine. A big drop during the day only points to the sun.