How to Clear a Green Pool Fast

The six steps

1. **Test cyanuric acid and pH before anything else.** Cyanuric acid above about 50 ppm means chlorine works less well at any given reading, and a pH above 7.8 means much of the chlorine you add converts to the weaker hypochlorite ion. Both change how much you need. A 7-way test strip covers both in fifteen seconds.
2. **Clean the filter and the pump basket.** Everything after this step depends on the filter, and a loaded filter is the reason more green-pool attempts fail than any chemical mistake. On a cartridge, rinse or replace. On a sand filter, backwash.
3. **Brush every surface, including the steps and behind the ladder.** Algae clinging to plaster is physically shielded from the water. Use a stainless brush on concrete and plaster, nylon on vinyl. This is the step people skip and the reason the shock underperforms.
4. **Shock to roughly 10 ppm free chlorine, dosed by weight for your volume.** Use the table below. Add it at dusk if you can, because sunlight destroys unstabilized chlorine fast. Pre-dissolve cal-hypo in a bucket if your pool has a vinyl liner. Products and chemistries are compared on best pool shock.
5. **Run the filter continuously - 24 hours a day, not on a timer.** Killing algae is chemistry and takes hours; removing it is filtration and takes days. Backwash or rinse the filter whenever pressure rises about 8 to 10 psi above clean.
6. **Clear the gray.** Once the green is gone the water usually turns cloudy gray, which is dead algae in suspension. That is success, not failure. Clear it with a clarifier if you can wait 24 to 48 hours, or a flocculant overnight if you can vacuum to waste.

How much shock, for your pool

The table below is computed, not copied. It assumes a 68% available chlorine calcium hypochlorite, which is the strongest common retail shock, and a target of 10 ppm free chlorine - a level appropriate to a visible bloom in a pool with normal cyanuric acid.

If you are using liquid chlorine instead

Liquid chlorine adds no calcium and no stabilizer, which makes it the better choice for hard water, for saltwater pools and for anyone already sitting near a cyanuric acid ceiling. It is sold by trade percent, and 12.5% is the common commercial strength.

How long each stage should take

If you are past those windows and still green, stop adding chemicals and check three things: cyanuric acid, filter condition, and whether the pump is genuinely running around the clock. In our experience of what the search data shows people ask afterwards, high cyanuric acid is the most common single cause, and no amount of extra shock fixes it - only dilution does.

What to do the day after it clears

• **Retest and rebalance.** Cal-hypo pushes pH up and adds calcium; trichlor pulls pH down. Either way the water is not where it was. Balance it before you swim.
• **Clean the filter properly, not partially.** It is holding a bloom's worth of dead algae, and leaving it there is how the next one starts.
• **Dose a polyquat algaecide now, not during the shock.** Chlorine consumes algaecide, so adding it while free chlorine is at 10 ppm mostly wastes it. See best pool algaecide.
• **Ask why it happened.** A pool that goes green once is unlucky. A pool that goes green twice has a cause: phosphate, stabilizer, or a filter that never recovered.