How to get algae out of a pool

Lower the pH, shock hard after sunset, brush everything and run the filter around the clock. That combination clears ordinary green algae in two to four days, and the step by step version is further down this page.

The fastest results come from doing all of it at once. Shock without brushing leaves algae protected on the walls, and shock without filtering leaves dead algae floating in the water, which is why so many green pools stall at cloudy.

Green algae clouds the water and wipes off the wall easily. Yellow or mustard algae hides in the shade, feels slightly slick and comes back in the same spot within days. Black algae is not really a smear at all: it is dark spots with roots dug into the plaster, and it laughs at a normal shock dose.

What you are looking at decides how much work this is. Green is a few days. Mustard takes longer and needs every surface brushed hard. Black usually needs the plaster treated rather than the water.

Because chlorine barely works at high pH. Above about 7.8 most of it is in a form that kills algae slowly, so shocking a pool with high pH burns money and clears nothing.

Get pH down near 7.2 before the shock goes in and the same dose does several times the work. The CDC gives 7.0 to 7.8 as the range for home pools. For a green out we aim for the low end of it, then bring it back to the middle once the water clears.

Roughly three times a normal shock dose, added after dark, and more if the stabilizer is high. The goal is to get free chlorine high enough to kill algae faster than it grows, and to hold it there until the water clears.

Stabilizer, also called cyanuric acid, is what usually throws people off. It protects chlorine from the sun, but the more of it there is, the more chlorine it takes to do the same job. That is why the CDC's minimum goes from 1 ppm to 2 ppm when stabilizer is in the water, and why a pool with very high stabilizer can seem to shrug off shock entirely.

Usually because the dose was not big enough to finish the job, the pH was too high, or the filter is not keeping up. If the water turned green but clear right after shocking, it may not be algae at all.

Clear green or teal water straight after a shock is often metal, copper or iron coming out of solution, especially on well water. More shock makes that worse. We cover how to tell the two apart in why is my pool green.

As a helper, yes. As the fix, no. Chlorine is what kills an algae bloom. A good algaecide added once chlorine has come back down can help stop it returning, but no algaecide clears a green pool on its own.

One caution for well water homes: some algaecides contain copper, and copper is one of the metals that stains plaster. If your pool already fights staining, use a copper free product.

If you have shocked twice, brushed daily, run the filter around the clock for four days and the water is still cloudy or the stain is still on the plaster, more chemical is not the answer.

At that point either the filter cannot keep up or the algae is in the surface rather than the water, and a drain and acid wash is often the cheaper end of it. Before you drain anything yourself, read should I drain my pool: in Florida that decision has a risk most people do not know about.

When it is past the point of doing it yourself, that is what a green pool cleanup is for.