Algae in Saltwater Pool

Algae grows in a salt pool for the same reason it grows in any pool. Free chlorine spent time below the level your stabilizer level demanded. The salt system is no defense against that, because it is a slow, steady producer and a bloom is a fast, large demand.

Getting rid of it means adding chlorine by hand and holding it high for days. The generator is not the tool for this job, and turning it up will not make it one. If your pool is green right now and you want the sequence rather than the explanation, start on green salt water pool.

Why a “self-chlorinating” pool gets algae at all

The phrase does a lot of damage. A salt system makes chlorine. It does not decide how much the pool needs, and nothing in it can tell whether the pool has any.

It converts dissolved salt to chlorine at a rate set by two things you control, the output percentage and the pump hours, and one you do not, the condition of the plates. Production is roughly flat across the day. Demand is not. Sun, swimmers, rain, heat and organic debris pull chlorine down in bursts, and a flat producer against a spiky demand runs a deficit at the worst moments.

The system also reports on itself badly. Most salt systems estimate salinity with an algorithm rather than measuring it, inferring it from how easily current crosses the cell, so scale, worn coating, cold water or the wrong cell type set in the menu all move that number without any salt moving. Nothing on the panel tells you your free chlorine. Only a test kit does.

The number that let the algae in

Trouble Free Pool explains the relationship: cyanuric acid protects free chlorine from the sun, so the level you must hold rises as CYA rises, and dropping below what your CYA requires is what gives you algae. They put recommended CYA for a salt water chlorine generator pool at 60 to 90 ppm, higher than a liquid chlorine pool, because the cell drips chlorine in slowly and needs it protected.

The numbers come from the FC/CYA chart, which reproduces that methodology as a table. For a salt pool the minimum free chlorine is about CYA multiplied by 5%.

Now put that next to Hayward’s maintenance chart, which asks for free chlorine of 1.0 to 3.0 ppm and stabilizer of 60 to 80 ppm with 80 the target. At CYA 80, Hayward’s entire chlorine band sits below the 4.0 ppm minimum on the chart.

That is why so many algae complaints start with “but my chlorine is fine.” It is fine by one chart and too low by another, and the algae is the tiebreaker. The fuller argument is on the CYA and chlorine chart. What is safe to say is that if you held 2 ppm and grew algae, 2 ppm was not enough for your pool.

What you are looking at

Three things get called algae and they do not behave the same. Nobody we would cite publishes kill concentrations by type, so treat these as recognition rather than dosing.

Green algae. The common one. Free floating, so it clouds the whole pool before it coats anything, taking water from clear to hazy to green in a couple of days. The most responsive to chlorine.

Yellow or mustard algae. Clings to shaded walls and the deep end, brushes off in a cloud, returns to the same spot. The one people fight three times before they hold chlorine high long enough.

Black algae. Dark spots rooted into plaster, under a surface layer chlorine does not easily reach. Brushing that layer open is not optional here.

Two lookalikes. A grey or brown dusting that vacuums straight up with no regrowth is dirt or pollen. A stain that will not brush off at all is metal, and chlorine makes metal staining worse.

Why the generator cannot clear a bloom

Owners reach for boost or super chlorinate because it is free and it is right there. Hayward’s manual describes super chlorinate as running the cell at full output for 24 hours. In The Swim states the ceiling: most salt systems in shock mode “will only raise the FC level by a few clicks - up to 5-7 ppm. It also does it very slowly, which will not have the desired effect of killing algae, removing chloramines or pathogens.”

Look at the chart again. A pool at CYA 80 needs 32 ppm to shock. Boost tops out around 5 to 7 ppm in a clean pool, and a pool full of algae consumes chlorine as fast as the cell makes it, so in practice the reading barely moves. You are running a slow race against something that reproduces. It also costs you: In The Swim notes that boost “places a heavy demand on the salt cell, shortening its lifespan,” and a cell runs $700 to $1,100 to replace.

The treatment: held, not dosed

Dose tables by pool size, and the adjustment for stabilizer, are on salt water pool shock. What matters here is the shape of the process, because that is where people fail.

Know your CYA first. It sets the shock target and everything after. In The Swim’s rule of thumb is that over 20 ppm CYA you add 50% more shock and over 40 ppm you double it, and a correctly run salt pool is past 40 by design.

Get pH into range first. Hayward’s band is 7.2 to 7.8. Chlorine works poorly at high pH, and salt pools drift up.

Balance nothing else. Calcium, alkalinity and salt can wait. Adding salt to a green pool fixes nothing, and if you do need it later, size it with the pool salt calculator.

Turn the generator down. It cannot help at these levels and wears the plates for no benefit.

Dose to shock level, then keep it there. This is the difference between success and a fourth weekend of this. Chlorine is consumed as it kills, so free chlorine falls all day. Test and re-dose back up, several times a day at first. One dose of the right size still leaves the pool below kill level within hours.

Brush daily and run the pump continuously. Chlorine cannot reach what is under a biofilm, and the filter is what removes the dead algae. Backwash or clean it when pressure rises.

Stop when three things are true at once. Free chlorine holds overnight with little loss, the water is clear, and nothing is returning to the walls. Losing a lot of chlorine overnight means something is still alive and eating it. Haze on the way out is normal, and cloudy salt water pool covers the tail end.

Expect days, not hours. A neglected green pool can take a week.

Then keep it away

Bring the generator back on, set the free chlorine you hold using your actual CYA rather than the number on the lid, and test weekly. If stabilizer has crept above your manufacturer’s range the whole ladder shifts up, and Orenda is blunt about the fix: draining and diluting is the most economical and practical way, reverse osmosis works and is expensive, and CYA does oxidize out on its own but “it takes months and high chlorine levels.”

Tabs are usually how it got there, trichlor being only 45.8% chlorine by weight with most of the rest cyanuric acid. What to stock instead is on salt water pool chemicals, and the rhythm that prevents all of this is salt water pool maintenance. Algaecide belongs in that conversation, not this one, and it helps less than the label suggests: see algaecide for a salt water pool.

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