Green pool recovery in Canada: from algae to crystal clear in 5 days

Every Canadian pool owner meets the green pool eventually, and it is almost always the same scene: the winter cover comes off in May and there is a swamp underneath. Sometimes it is a July heat wave that outruns your chlorine instead. Either way, the fix is the same. No algaecide shortcut, no magic powder — just chlorine, brushing, and pump runtime.

Day 0: assess and balance

Before you spend a dollar on chemicals, test the water. The two numbers that decide your fight are cyanuric acid (CYA, sold as stabilizer or conditioner) and pH.

If CYA is over 60 ppm — common if you ran chlorine pucks all last season — drain a third of the water and refill before you start. Check your municipality's rules first, since many Canadian municipalities have bylaws about where pool water can be discharged. You cannot win against algae if chlorine cannot reach its shock level.

If pH is over 7.6, lower it to 7.2 with muriatic acid from the pool store. Chlorine kills much faster at slightly acidic pH.

Clean the filter completely. A dirty filter will choke on dead algae within 12 hours.

One cold-water tip that saves Canadians real money: open the pool early. While the water is still below about 15°C, algae barely grows and recovery is fast and cheap. The worst swamps belong to pools opened late in June.

Day 1: SLAM (shock level and maintain)

SLAM means raising free chlorine (FC) to ten times your CYA level and keeping it there. So 30 ppm CYA = 30 ppm free chlorine target.

Doses assume 10.3% liquid chlorine, the strength most Canadian pool stores sell in refillable carboys — check your label and scale for your own pool's volume.

Add the chlorine in the evening. Brush every wall and the floor. Run the pump 24 hours a day until this is over.

Day 2: retest and recharge

Test free chlorine in the morning. It will have dropped a lot — that is the algae dying. Bring FC back up to the shock target, brush again, and keep the pump running.

The water will turn from green to grey. Grey is dead algae. You are winning.

Days 3 to 4: clean the filter, keep brushing

Backwash or rinse the filter every 12 to 24 hours — it is doing the heaviest work of its season. The water will go cloudy and milky before it clears. Hold chlorine at shock level. Keep brushing.

Day 5: the overnight chlorine loss test

When you think you are done, prove it. At sunset — and in a Canadian June that means late, so be patient — record free chlorine. Before sunrise, test again. If you lost less than 1 ppm overnight and the water is clear, the algae is dead. Let chlorine drift down to 1–3 ppm and resume normal care.

If you lost more than 1 ppm, hold shock level another day and retest.

What about algaecide

Polyquat 60 algaecide is a useful preventive at spring opening. It is not a fix for an active green pool — added to a swamp it mostly wastes money and can cause foaming. Save it for after recovery.

When to call a pro

If the pool will not respond after five days of correct SLAM, you may have mustard algae, black algae, or a phosphate problem feeding new growth as fast as you kill it. A pool service can identify and treat it — get two or three quotes, because a full recovery usually means multiple visits.

Green pool FAQs

Will algaecide alone clear a green pool? No. Algaecide is a preventive. Once a pool is green, you need chlorine at shock level for several days.

How much chlorine will I need? For a 50,000 L pool with a moderate bloom, plan on three or four 20 L carboys of liquid chlorine over the five days. Price a refill at your local pool store before you start — refills are far cheaper than jugs.

Can I swim before it is fully clear? No. Cloudy water hides the bottom, which is a drowning hazard, and shock-level chlorine is rough on skin and swimsuits. Wait for full clarity and FC below 5 ppm.

Do I need to drain the pool? Only if CYA is over 60 ppm, or the pool is such a swamp that chlorine would cost more than a refill. Most green pools recover without draining — and mind your local discharge bylaws if you do.

Why did my pool turn green overnight? Free chlorine hit zero. Common causes: a heat wave plus a busy weekend, a thunderstorm that diluted everything, a salt cell that quietly failed, or high CYA blocking your chlorine. Find the cause or it will happen again.

How do I avoid the green opening next spring? Close late and open early — cold water is your friend at both ends. Balance the water before closing, and get chlorine tested and circulating the same day the cover comes off.

A clear pool is worth real money in a short season

Canada's swim season is short, which is exactly why hourly pool rental works here — every clear, sunny week counts. Pool Rental Near Me lets you list your pool free, and founding hosts pay 0% host fees, forever — you keep 100% of your price. Prices on the site currently display in US dollars, and hosts are paid out in Canadian dollars via Stripe. For context from the more mature US market: top backyard pools in Southern California earn $800–$1,000 in a good summer week. That is US data, not a Canadian promise, but it shows what peak-season demand can look like.

Start with our guide to renting out your pool in Canada, see how hourly rental works in Toronto, and once your water passes the overnight test, List your pool.