How Do I Get Rid of Green Algae in My Pool?

Getting rid of green algae takes four things done together: correct pH, hard brushing, chlorine held at shock level continuously (not once), and a filter running nonstop. Skip any one of them and the bloom comes back within days. A single bag of shock dumped in at sunset is the most common failed attempt we get called out to fix.

Step one: test and fix pH before you shock

Test free chlorine, pH, total alkalinity and cyanuric acid. Bring pH down to 7.2 to 7.4 before adding chlorine. At high pH most of your chlorine sits in a form that barely kills anything, so shocking an 8.0 pool wastes a good portion of what you add.

Total alkalinity should land between 80 and 120 ppm. If alkalinity is very low, pH will not hold still and you will be chasing it all week.

Step two: brush everything, then keep brushing

Algae anchors to surfaces and shields itself. Brushing breaks the colony open so chlorine can reach the cells.

• Use a stainless bristle brush on plaster and gunite, nylon on vinyl and fiberglass
• Cover walls, floor, steps, the light niche, behind the ladder and the tanning ledge
• Brush at least once daily for the whole treatment, not just on day one

Step three: raise chlorine and hold it there

Your target free chlorine is roughly 40 percent of your cyanuric acid reading.

• CYA 30 ppm: hold about 12 ppm free chlorine
• CYA 40 ppm: hold about 16 ppm
• CYA 50 ppm: hold about 20 ppm

For dosing, one gallon of 12.5 percent liquid chlorine raises 10,000 gallons by about 12 ppm. One pound of 73 percent cal hypo raises 10,000 gallons by roughly 8 to 9 ppm (skip cal hypo if calcium hardness is already high). Test and re-dose back to target two or three times a day. This is the part people quit too early.

Run the pump 24 hours a day. Clean or backwash the filter whenever pressure rises 8 to 10 psi over its clean baseline, and expect to do that repeatedly as dead algae loads it up.

Step four: know when you are done

You are finished when all three are true: free chlorine drops no more than 1 ppm overnight, the water is clear, and there is no visible algae anywhere. Then let chlorine drift back to normal and vacuum the settled debris to waste. If the water is clear but dull, a clarifier helps the filter grab the fine dead cells.

When to stop and call someone

Call a professional if the pool will not hold chlorine after a week, if CYA is above 80 ppm (a partial drain is the only real fix), or if the algae is dark and scrubs back the same day, which usually means black algae rather than green.

The Keys reality on algae pressure

In Key West there is no freeze to reset the season, so algae never gets shut down the way it does up the coast. Year-round UV also strips chlorine fast, which is why a pool here can go from clear to green in under 72 hours after a pump failure. Salt-laden air and storm debris add organic load on top of that. Treating an outbreak is straightforward; the discipline is in never letting free chlorine sag between visits.

Write your target chlorine number on the equipment pad so whoever tests the water knows what “enough” actually means for your CYA level.