Killing Green Algae Is the Easy Half. Clearing It Is Not

Green algae is the ordinary case, and it is a sanitizer failure

Green algae turns the whole body of water rather than marking parts of it. The pool clouds, then tints, then goes opaque, and it happens everywhere at once because the algae is suspended in the water rather than attached to the wall. That is what separates it from the two harder cases, and the difference decides the treatment.

If instead you have flat yellow patches on the shaded wall that brush away and return by morning, that is mustard algae, which is being reintroduced from something outside the water. If you have small dark spots that catch a thumbnail and survive a correct dose, that is black algae, which is anchored into the surface. Both need something this page does not cover. Green algae needs chlorine, a brush and time.

The cause is almost always the same: free chlorine reached zero and stayed there long enough for what is always in the water to multiply. So the treatment is not a product. It is getting the sanitizer back up, keeping it there, and then waiting out the filter.

Correct the pH before you dose

Chlorine does more work per ounce at the lower end of the pH range than at the top of it, so a pool sitting high is a pool where the dose you are about to add underperforms. Test pH and bring it into range first, then chlorinate. The sequence when several readings are wrong at once is covered in pool chemistry in the right order, and it is worth following, because correcting things in the wrong order is how an afternoon becomes a week.

Skim the surface and net out leaves and debris before you start. Organic material consumes chlorine that you wanted spent on the algae.

What to add, and how much

Our pool chlorine calculator works from the amount each product needs to raise free chlorine by 1 ppm in 10,000 gallons, then scales it to your volume. For a 20,000 gallon pool it gives:

A green pool normally tests at zero free chlorine, so bringing that same pool up to 4 ppm asks for 81.6 fl oz of the 12.5% liquid, a little under two thirds of a gallon. Every further ppm you want on top of that is another 20.4 fl oz of the same jug.

What target to aim at is not a number this page will hand you, because it depends on your stabilizer reading and on the label of the product in your hand. What the calculator gives you is the quantity for whatever target you settle on, in the units printed on the bottle. Note that it assumes the strength on the label, and chlorine weakens as it sits on a shelf, so an old jug delivers less than a fresh one.

All of it rests on knowing your volume. If yours has always been an estimate, the pool volume calculator derives it from your dimensions, and an estimate that is off by a fifth makes every dose you ever add off by a fifth with it.

Brush the entire pool after dosing, walls, floor, steps and behind the ladder. Brushing breaks up the colonies and puts them where the chlorine is.

The step nobody times: filtration

Here is the part the other results leave as "run the filter until it clears".

Our guide to pump run time gives the turnover formula: pool gallons divided by flow rate in GPM times 60. A 20,000 gallon pool with a pump moving 40 GPM turns over in 8.3 hours. That is one pass of the whole pool through the filter.

Now look at what a normal schedule buys you. The same guide describes one turnover a day as the routine maintenance baseline, which is exactly right for a pool that is already clear and entirely inadequate for one that is not. Left on that schedule, a pool full of dead algae gets one pass every twenty four hours.

Run the pump continuously instead and that same pool gets a shade under three passes a day. That is the whole difference between clearing over a weekend and clearing over a fortnight, and it costs electricity rather than chemicals. Leave the pump on, around the clock, until the water is clear rather than until the clock says stop.

Expect the water to turn cloudy grey or milky blue as the algae dies. That is the treatment working, and it is filter load rather than a new problem.

When it stays green

Chlorine holding but water still not clearing points at the filter rather than the chemistry.

• The filter is blinded. Dead algae loads media fast. Watch the pressure gauge and backwash or clean the cartridge when it climbs, then keep running.
• The pump is short cycling on a timer that overrides you. Check it is actually running all night.
• Debris was never removed. Vacuum the settled layer to waste where your equipment allows it, rather than pushing it back through the filter.
• Chlorine keeps falling to zero by morning. That is a stabilizer or a demand problem, not an algae problem, and the pool will go green again as soon as you stop.

Fix the reason the chlorine reached zero and this stops being an annual event. Treat only the colour, and you will be doing it again next season.