Why Is My Pool Green?
Short answer
A pool turns green when free chlorine falls below the level needed to kill algae, letting a bloom take over. The usual triggers are a chlorine outage, cyanuric acid so high it neutralizes the chlorine you have, high pH, or a pump and filter that are not running long enough.
Your pool is green because algae is growing in it, and algae only grows when free chlorine has dropped below the level it takes to kill it. The color is a symptom. The actual problem is a sanitizer failure, and until you find which one, shocking the pool will only buy you a few days.
What “not enough chlorine” actually means
Free chlorine is not judged on its own. It has to be read against your cyanuric acid (CYA), the stabilizer that protects chlorine from sunlight. As a working rule, free chlorine needs to sit at roughly 7.5 percent of your CYA level just to hold algae off.
- CYA 30 ppm: keep free chlorine at 2 to 3 ppm minimum
- CYA 50 ppm: keep free chlorine at 4 ppm minimum
- CYA 80 ppm: you now need 6 ppm or more, all the time
That is why a pool can test at “2 ppm chlorine” and still go green. If CYA has crept to 90 ppm from years of trichlor tablets, 2 ppm is functionally zero.
The four causes we find most often
Chlorine ran out. The feeder emptied, the tab floater ran dry, or the salt cell stopped producing. A few hot days with no sanitizer is all it takes.
CYA is too high. Trichlor tablets add about 6 ppm of CYA for every 10 ppm of chlorine they deliver. Over a season it stacks up. The only practical fix is draining and replacing water, since nothing removes CYA chemically.
pH drifted high. At pH 7.2, roughly 60 percent of your free chlorine is the active killing form. At pH 8.0 it is under 25 percent. Same test result, a fraction of the power. Keep pH at 7.4 to 7.6.
The pump is not running enough. Water that does not move does not get sanitized. Steps, corners and behind ladders bloom first for exactly this reason.
How to clear it
Test first, then brush every surface, then raise free chlorine to about 40 percent of your CYA level and hold it there. Retest and re-dose two or three times a day until the water stops consuming chlorine overnight. Run the filter around the clock and backwash or clean when pressure climbs 8 to 10 psi above its clean reading. Most green pools clear in three to five days. A pool you can no longer see the first step in can take one to two weeks.
What this looks like in the Florida Keys
There is no off-season here. Mainland pools get a cold stretch that stalls algae for months. Key West does not, so algae pressure is a year-round, 12-month condition. Add UV that burns off unstabilized chlorine in a matter of hours and summer storms that dump organic debris and dilute your water, and the margin for a missed week is much thinner than the internet assumes.
There is also a public health angle that is specific to this island. Standing green water is prime Aedes aegypti breeding habitat, and mosquito-borne illness is a live local concern, not a theoretical one. A green pool is a neighborhood problem here in a way it is not in Denver.
If your pool has gone green more than once in a season, stop treating the color and get a full test on CYA, pH and total alkalinity, because something in the chemistry is fighting you.
People also ask
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