Pool algae: brush, shock, filter, then rebalance

First: is it actually algae?

Pollen is the impostor, especially in spring. Two tells. Pollen tends to sit as a yellow-green film on the surface and along the edges, and it wipes onto your hand rather than staining. Algae grows on surfaces — the walls, the floor, the waterline — and feels slippery.

The definitive test: brush a patch of wall. Algae comes off as a cloud that re-clouds the water. Pollen does not. If it is pollen, the answer is filtration and a skimmer net, not chemicals.

Why algae happened

Almost always one of three things, and knowing which stops it recurring.

Free chlorine fell. The CDC publishes a minimum of 1 ppm free chlorine, or 2 ppm with a stabilizer. Below that, algae has an opening. A heatwave, a heavy bather day or a week away will all do it.

Cyanuric acid climbed too high. Every trichlor tablet adds some. In a pool that gets topped up rather than drained, it accumulates until your chlorine reading looks fine and the chlorine cannot actually do anything. This one is invisible without a test strip that reads cyanuric acid.

The filter was not keeping up. Under-filtered water in summer heat gives algae somewhere to establish before chlorine reaches it.

The sequence

1. 1

   Brush every surface, thoroughly

   This is the step people skip and it is the one that decides whether the shock works. Algae growing on a wall is protected by its own biofilm; chlorine cannot reach it until you physically break that up. Brush the walls, the floor and especially the waterline and any corners.
2. 2

   Test, and correct pH first

   Shock works poorly at high pH. Get pH into the CDC's published 7.0–7.8 range before you shock, or you will be paying for chemical that cannot act.
3. 3

   Shock hard, once, with unstabilized shock

   Use calcium hypochlorite rather than a stabilized dichlor: cal-hypo adds no cyanuric acid, and high cyanuric acid is frequently why the bloom happened. Dose for your full volume, at night if possible so sunlight does not destroy it before it works. Under-dosing is the single most common failure — more on getting the dose right.
4. 4

   Run the filter continuously

   24 to 48 hours without stopping. Dead algae is fine particulate and it has to physically come out. Expect the water to go from green to cloudy gray before it goes clear — that transition is the sign it is working.
5. 5

   Clean or replace the filter, repeatedly

   The cartridge will load heavily and fast. Rinse it, keep going, and expect to replace it after a serious bloom. A blocked filter at this stage stalls the whole recovery.
6. 6

   Vacuum the settled debris

   Dead algae settles on the floor. Vacuum it out rather than waiting for the filter to catch it, then clean the filter again.
7. 7

   Rebalance, then add algaecide to prevent recurrence

   Re-test everything, bring alkalinity, pH and chlorine back into range, and only then add a polyquat algaecide as prevention. If cyanuric acid was the root cause, a partial drain and refill is the only way to lower it.

Algaecide: prevention, not cure

Polyquat algaecide is genuinely useful and it is widely misused. It is good at stopping algae establishing and poor at killing an established bloom. Use it as part of the rebalance at the end of the sequence above, and as part of closing the pool for winter, which is where it earns its place most clearly.

Choose polyquat over copper-based algaecides for a vinyl-lined pool: copper can stain a liner permanently, and the staining costs vastly more than the few dollars the cheaper product saved.

Keeping it away

Three habits. Hold free chlorine at or above the published minimum rather than letting it drift and rescuing it. Read your cyanuric acid every few weeks — if it is climbing, a partial drain and refill is the only fix. And brush the waterline weekly, because that is where it always starts. The full routine is in the beginner’s guide.