Why Does My Pool Keep Turning Green Even With Chlorine in It?

If your test shows chlorine and the pool still goes green, the chlorine is present but not working. In almost every case we see, that comes down to cyanuric acid being too high, pH being too high, or water that is not actually circulating. All three produce the same frustrating result: a test strip that looks fine over water that is not.

Cause 1: cyanuric acid has climbed too high

This is the number one answer. Cyanuric acid protects chlorine from sunlight, but it also binds it. The more CYA in the water, the more free chlorine you need before any algae dies.

You need free chlorine at roughly 7.5 percent of your CYA level just to hold algae off:

• CYA 40 ppm: you need at least 3 ppm free chlorine
• CYA 80 ppm: you need at least 6 ppm
• CYA 120 ppm: you need at least 9 ppm, all the time

So a pool reading 3 ppm at CYA 120 is, in practical terms, an unsanitized pool.

CYA creeps up because trichlor tablets add about 6 ppm of CYA for every 10 ppm of chlorine they deliver, and dichlor is worse. Nothing chemically removes CYA. The only fix is replacing water. Keep CYA between 30 and 50 ppm for a chlorine pool and 60 to 80 for a salt pool.

Cause 2: pH is too high

At pH 7.2 roughly 60 percent of your free chlorine exists as hypochlorous acid, the form that actually kills. At pH 8.0 it is under 25 percent. Same reading on the test, a fraction of the effect. Keep pH at 7.4 to 7.6.

Cause 3: you are reading the wrong number

Test strips and cheap kits often report total chlorine, which includes combined chlorine (used-up chlorine that no longer sanitizes). A pool can show 3 ppm total with almost no free chlorine. Use a FAS-DPD test kit that reads free chlorine separately, and if combined chlorine is above 0.5 ppm, breakpoint chlorination is needed, which means raising free chlorine to about 10 times the combined reading.

Cause 4: dead circulation

Algae blooms where water does not move. Check for a clogged impeller, a dirty filter, closed or misaimed return jets, a failed variable-speed schedule running too slow, or simply too few pump hours. Steps, corners, tanning ledges and behind ladders are where it shows first.

Cause 5: it is not green algae

Mustard algae tolerates chlorine levels that kill green algae outright, and black algae shrugs it off entirely behind a protective layer. If the growth is yellow and brushes off in a cloud, or dark and rooted in the plaster, you have a different problem requiring a different treatment.

The Keys angle

Key West pools drift into high CYA faster than most because owners here compensate for extreme UV with heavy stabilizer and tablet feeders, and there is no winter shutdown to interrupt the accumulation. We routinely test Keys pools at 100 ppm CYA or more, which is why “I keep adding chlorine and it keeps going green” is such a familiar call on this island.

Water replacement is also a bigger deal here since our fill water comes down one pipe from the mainland, so we plan partial drains deliberately rather than casually.

Before you buy more chlorine, test cyanuric acid, because that single number explains most of these cases.