How to clear a green pool

Green water is algae, and algae shows up when chlorine runs low for too long. Hot weather, sun and weak circulation make it worse. The fix is the same every time: get the chemistry right, physically knock the algae loose, flood the water with chlorine and let the filter haul it all out.

It's work, but it's rarely a reason to drain the pool.

Why pools turn green

Low free chlorine is the main cause. South Australia's health department says algae means free chlorine "is not being properly maintained," and California's Monterey County lists "insufficient chlorine." Sun and warm water speed algae up, and so do dead spots where water barely moves, like corners, steps and behind ladders.

Phosphates come up a lot online. They can feed algae, but CDC's pool code notes that "peer-reviewed data is lacking." Treat them as a side issue. Chlorine first.

Figure out which algae you have

Plain green algae floats in the water and turns it cloudy green. It's the most common type and the easiest to kill.

Mustard algae is yellow-brown and clings to walls. People mistake it for pollen or dirt. Clorox calls it "resistant to chlorine," and it tends to come back.

Black algae forms dark spots, usually on plaster, with a tough protective crust. Brushing it hard matters more than anything else.

Step 1: test and fix pH first

Chlorine works much better at normal pH. At pH 7.5 about half of your chlorine is in its strong, germ-killing form. At pH 8.0 it's roughly a quarter, according to CDC's code. The EPA-registered HASA Algi-Ban label says to get pH to 7.2–7.6 before treating, and Monterey County gives the same range.

If pH is high, lower it before you shock. Use the pH calculator for the amount of acid. Wear goggles and gloves, add the acid with the pump running and let it circulate before any chlorine goes in. The CDC says never to mix acid and chlorine products, and to keep them apart in storage too.

Step 2: clean the filter, then brush everything

Start with a clean filter, because it's about to work harder than it has all season. Backwash a sand or DE filter, or hose off the cartridge.

Then brush every wall, step and the floor, and don't skip the corners. Brushing breaks up algae so chlorine can reach it. For black algae on plaster, the Green Treat label says to use a stainless steel brush. On vinyl, fiberglass or painted pools, use nylon only. Steel bristles can damage those surfaces.

Step 3: shock well above normal

You need far more chlorine than usual to kill an algae bloom, and sources don't agree on one number. County health departments say 5 to 10 times your normal level. Taylor, the test kit maker, uses 30 ppm for algae. Your shock product's label gives its own dose, so start there.

Here's what raising chlorine from zero takes in a 15,000-gallon pool:

Target chlorine	10% liquid chlorine	65% cal-hypo
10 ppm	1.5 gal	31 oz
20 ppm	3 gal	3 lb 14 oz
30 ppm	4.5 gal	5 lb 12 oz

Your pool isn't 15,000 gallons? Put your own size into the shock calculator. Shock in the evening, as HTH recommends, so the sun doesn't burn the chlorine off before it works.

One warning about testing: very high chlorine can bleach a standard color test so it reads zero. The CDC says this can happen above 10 ppm. If your kit reads 0 right after a big dose, don't trust it. An FAS-DPD drop-count kit avoids color matching and reads up to 20 ppm, according to Taylor and Pool & Spa News.

Step 4: run the filter nonstop

Run the pump and filter 24 hours a day until the water is clear. Clorox and Leslie's both say this. Dead algae clogs filters fast, so check the pressure gauge and clean the filter whenever it climbs 8 to 10 psi above its clean reading.

The water may turn cloudy as the algae dies. Keep filtering; it clears as the filter catches up.

Step 5: vacuum, retest and repeat

Once the water starts to clear, dead algae settles on the floor. Vacuum it out. If there's a lot, vacuum to waste (the drain setting on a sand or DE filter) so you don't push it straight back through the filter.

Then test again. If chlorine dropped to zero overnight, there's still algae eating it. Brush and shock again. Keep repeating until the water is clear and chlorine holds steady from evening to morning.

When can you swim again?

Two conditions. First, the water has to be clear enough to see the bottom. CDC's pool code closes public pools when the main drain isn't visible from the deck, because a swimmer in trouble can't be seen. Use the same rule at home.

Second, chlorine has to drop back to the swim limit on your shock product's label. On the HASA liquid chlorine label it's 4 ppm for pools; the Green Treat label says 3. Check yours.

Keep it from coming back

Hold free chlorine in range every day, run the filter long enough to turn the water over, and brush the shady spots weekly. If algae keeps returning even though you're adding chlorine, test your stabilizer (CYA). Too much makes chlorine slow to kill, and only draining some water brings it down. The ideal levels chart has the targets.