A Green Pool: What to Do First

Green water is algae winning a race against your chlorine, and the fastest way back is chlorine winning it instead — not a bottle of algaecide poured in as a first move. This page sets out the order that actually clears a pool: what to check, what to physically remove before you dose anything, how much chlorine the water needs, and what to run afterward while it works.

First, test — don't guess the dose

Free chlorine, combined chlorine, pH and cyanuric acid all change what a green pool actually needs. A pool that has been green for a week with no chlorine reading at all needs a different plan than one that greened up overnight with combined chlorine already climbing. The pool chemical calculator and the shock calculator both start from these same four readings, so the testing step is not optional groundwork — it's the input the rest of the plan runs on.

Brush before you shock

Algae that has attached to a wall, a step or a crack will not clear just because the surrounding water goes clean. Michigan State University Extension's manual, written for pool operators, describes how the different colors behave: the common free-floating green kind is the easiest to clear with chlorine alone, while black and yellow algae attach to walls, floors and steps and are difficult to remove once they've taken hold there. Black algae in particular needs mechanical help — the manual describes brushing it with a stainless-steel brush to break open the colony's outer wall so the chlorine already in the water can actually reach it. Brushing first gives the dose that follows something to work on, instead of leaving a colony sealed under its own surface.

What algae actually does to the water

The same manual lists what a bloom does to the water beyond its color: it turns the water turbid and dirty-looking, makes the surfaces in and around the pool slippery, and clogs a filter that's trying to pull it out. A pool too cloudy to see the bottom of is not just unpleasant — it is the same test the Model Aquatic Health Code (a public-pool code) uses to call a public venue unsafe to swim in: an "AQUATIC VENUE bottom not visible" is one of that code's own imminent hazards, a public-pool standard rather than a rule for anyone's back garden, but a useful line for judging when a green pool is still too cloudy to use.

Dose to what the water needs, not a flat number

A pool this far gone usually needs more than a top-up to the everyday target. The shock calculator works out the larger of the gap to your shock target and ten times your combined chlorine reading — the breakpoint dose that clears chloramine haze along with the algae — and shows the free chlorine you'll be left with afterward. For heavy accumulations, Michigan State University Extension's manual describes what a public pool falls back to when brushing and a normal dose aren't enough: draining the pool and scrubbing every exposed surface with a strong chlorine solution mixed specifically for that job. That is a last resort for a pool past what dosing and brushing can fix, not a routine step, and this site's calculators size everyday doses, not that one.

Run the filter, then retest

Chlorine alone does not clear cloudy water — dead algae still has to be filtered out. Run the pump and filter continuously rather than on a normal schedule until the water is visibly clear, backwashing or cleaning the filter as it loads up, since a filter clogged with dead algae stops doing its job halfway through the recovery. Retest free chlorine, combined chlorine and pH along the way; a second, smaller dose is common once the first round of chlorine gets consumed clearing the worst of it.

Where algaecide fits

An algaecide is not a substitute for the steps above — it is what you reach for once chlorine and brushing have done the work, if a particular strain keeps coming back. The chemical calculator and the shock calculator size every chlorine dose in this order; neither tool doses an algaecide, so follow its own product label for that part of the job.