Why does my Ottawa pool turn green overnight even after shocking it?

An Ottawa pool that turns green overnight after shocking typically has one of four underlying problems: cyanuric acid (stabilizer) levels that are too high and locking out chlorine effectiveness, a phosphate level feeding algae faster than shock can kill it, insufficient filtration run time that leaves dead zones where algae survives, or a shock dose that was too weak for the severity of the contamination. Shocking alone does not fix a green pool if the root cause remains unaddressed.

The most common culprit in Ottawa pools is over-stabilization — a condition where cyanuric acid (CYA) levels have crept above 80 to 100 ppm over the course of the season or over multiple seasons. Cyanuric acid is the stabilizer that protects chlorine from UV sunlight degradation, and it is present in every trichlor tablet and dichlor granule you add to the pool. The problem is that CYA does not break down or evaporate. It accumulates. Each time you add stabilized chlorine, the CYA level rises slightly. By mid to late summer in Ottawa — after 10 to 14 weeks of continuous trichlor tablet use — CYA can reach 100 to 150 ppm or higher, especially in pools that have not had significant water replacement through backwashing or splash-out.

At high CYA levels, chlorine becomes increasingly ineffective. The relationship between CYA and free chlorine is not linear — it is exponential. At 30 ppm CYA, a free chlorine level of 2 ppm provides excellent sanitation. At 100 ppm CYA, you need a free chlorine level of 7 to 8 ppm to achieve the same sanitizing power. Most pool owners and even some service technicians test free chlorine, see a "normal" reading of 2 to 3 ppm, and assume the water is properly sanitized — but with high CYA, that chlorine level is functionally useless against algae. You shock the pool, briefly raise chlorine to 10 ppm (which is still inadequate relative to the CYA level), and within hours the algae rebounds because the chlorine was never truly effective.

The only reliable fix for over-stabilization is partial water replacement. Drain 25 to 40 per cent of the pool water and refill with fresh municipal water, which contains zero CYA. In Ottawa, this is straightforward during summer when garden hose water is readily available and warm enough not to shock the vinyl liner. After refilling, test CYA and confirm it has dropped below 50 ppm before resuming normal chlorination. Going forward, consider switching to unstabilized chlorine (liquid sodium hypochlorite or calcium hypochlorite shock) for your primary sanitizer, and add CYA separately only as needed to maintain 30 to 50 ppm.

Phosphates are the second major reason Ottawa pools turn green despite shocking. Phosphates are a nutrient that feeds algae, and they enter your pool through municipal fill water, lawn fertilizer runoff, decomposing leaves and pollen, and even some pool chemicals. Ottawa's mature urban tree canopy drops enormous quantities of organic material into pools throughout summer, and the heavy clay soils in neighbourhoods like Barrhaven, Kanata, and Orleans contribute phosphate-laden runoff during rainstorms. If your phosphate level exceeds 300 to 500 parts per billion (ppb), algae has a ready food source and can regrow rapidly after shocking.

Phosphate removers are available at Ottawa pool stores for $20 to $40 per treatment dose. These products bind to phosphates and precipitate them out of solution, where your filter captures them. After treatment, backwash or clean the filter thoroughly. Note that phosphate removal is a treatment, not a one-time fix — phosphates continuously enter the pool from environmental sources, so periodic testing and treatment throughout the season keeps levels manageable.

Insufficient filtration is the third common cause of overnight greening in Ottawa pools. Your filter system can only kill algae (via chlorinated water passing through the filter) and remove dead algae cells when it is running. Many Ottawa pool owners run their pumps only 6 to 8 hours per day to save on electricity costs. During peak summer heat, when water temperatures exceed 27°C and algae growth accelerates exponentially, 8 hours of filtration may not turn over the entire pool volume even once — the minimum standard is one complete turnover per day, and two turnovers is recommended during heat waves. A standard residential pool with a 1.5 HP pump circulates approximately 4,500 to 6,000 litres per hour, meaning a 50,000-litre pool needs 8 to 11 hours of run time for a single turnover.

After shocking a green pool, run the filter continuously — 24 hours a day — until the water is clear. This is non-negotiable. Shocking raises the chlorine level, but the filter must physically remove the dead algae cells that make the water cloudy and green. Brush the walls and floor to dislodge algae into suspension where the filter can capture it. Backwash or clean the filter every 12 to 24 hours during the recovery process, because the filter will clog rapidly with dead algae. The recovery typically takes 2 to 4 days of continuous filtration, multiple backwashes, and possibly a second or third shock treatment.

Finally, the shock dose itself may have been insufficient. To kill an active algae bloom, you need to reach "breakpoint chlorination" — a free chlorine level at least 10 times the combined chlorine reading, or roughly 30 ppm for a visibly green pool. One bag of calcium hypochlorite shock raises a 40,000-litre pool by approximately 8 to 10 ppm. A genuinely green pool may require 3 to 4 bags at once — a dose that surprises homeowners accustomed to adding a single bag for weekly maintenance shocking. Under-dosing is worse than not shocking at all because it stresses the algae without killing it, potentially selecting for chlorine-resistant strains.

Struggling with a persistently green Ottawa pool despite regular shocking? Ottawa Pool Installation connects homeowners with experienced local water chemistry specialists who can diagnose the root cause and restore your pool to clear, swimmable condition.