What causes white flakes floating in my Ottawa pool and how do I get rid of them?

White flakes floating in your pool are almost always one of three things: calcium carbonate scale flaking off your pool surfaces, degraded bichloride or chlorine tablet residue, or — most commonly in Ottawa pools — white mould or "white water mould" combined with a biofilm called pink slime. Identifying which one you are dealing with determines the fix.

Calcium scale flakes are the most common culprit in Ottawa, particularly in pools that have been running through a few seasons. Ottawa's municipal water supply has moderate hardness, and when calcium hardness climbs above 400 ppm — which happens gradually through evaporation and chemical additions — calcium carbonate precipitates out of solution and forms a white, chalky scale on pool surfaces. When that scale builds up and then flakes off, you get white particles floating in the water. To confirm this, rub a flake between your fingers — calcium scale feels gritty and slightly rough. The fix involves lowering your calcium hardness (partial water replacement is often the most practical approach), adjusting your pH down to the 7.2 to 7.4 range, and using a sequestering agent or scale inhibitor. A proper water chemistry test from a pool supply store — not just a basic strip test — will tell you exactly where your calcium hardness, pH, and total alkalinity sit.

White water mould is a naturally occurring fungal organism that thrives in pools with inconsistent chlorine levels, and Ottawa's shoulder season creates ideal conditions for it. When you open the pool in late May or close it in September, water temperatures drop and chlorine becomes less effective — mould can establish itself quickly. White mould looks like shredded tissue paper or wet cotton floating in the water. Getting rid of it requires a sustained shock treatment — typically triple-shocking the pool (bringing chlorine to 10 ppm or higher), brushing every surface aggressively including steps, ladders, and any crevices, cleaning or replacing your filter media, and maintaining elevated chlorine for several days until the mould is completely gone. It often takes two or three rounds of treatment to fully eliminate it.

Degraded tablet residue is less common but worth checking — if you use trichlor tablets in a floating dispenser or skimmer basket, undissolved or partially dissolved tablet fragments can occasionally break apart and float. This is more of a nuisance than a water quality problem, but switching to a proper tablet feeder or adjusting your dosing schedule usually solves it.

One important Ottawa-specific caution: if you are seeing white flakes shortly after opening the pool in spring, and the pool was closed with a solid tarp rather than a mesh safety cover, you may be dealing with a combination of mould, algae die-off, and accumulated debris that has been sitting under the cover all winter. Ottawa's long, cold winters give mould and biofilm plenty of time to establish in a closed pool. A thorough spring shock protocol is your best defence — and getting a full water chemistry panel done at a local pool supply store before you start adding chemicals is always the right first step.