A green pool is never a welcome sight, but especially not for commercial, public, or community facilities. Swimmers notice it immediately, health inspectors ask questions, and sudden closures can disrupt your operations. In other words, it’s an issue that requires your immediate attention.
Understanding why your pool has turned green—and how to respond—starts with knowing what the underlying cause is, as well as your current water parameters. Most often, green water means either algae growth or dissolved metals, though sometimes both are involved. From there, you can take steps to transform pool water so it’s clear again. Regular and accurate testing and maintenance can then help you prevent it from becoming a recurring problem that turns guests away.
Ahead, we’ll walk you through the possible causes, how to diagnose which one you’re dealing with, and the appropriate treatment steps.
Algae is the leading cause of green pool water. The danger is, even when chlorine levels appear normal, algae can grow if chlorine’s sanitation abilities aren’t fully effective. This is typically the case when cyanurunic acid (CYA), or pool stabilizer, levels are too high. CYA stabilizes chlorine against sunlight, but excessive amounts reduce its ability to disinfect, a phenomenon commonly known as “chlorine lock”. Pool pH or alkalinity that are out of range can also cause issues with chlorine’s ability to sanitize, and subsequently allow algae growth.
Common types of algae include mustard (yellow) algae and black algae, but if your pool water is green, green algae is your likely culprit. Green algae both floats freely or lightly coats pool surfaces.
However, not all green pools are due to algae. Metals like copper, iron, and manganese can discolor water when oxidized by chlorine.
Metals can enter pools from:
Unlike algae, metals won’t grow or spread; they’re already in the water. Treating a metal-induced green pool requires metal sequestrants or filtration, rather than relying on chlorine shock alone.
First, you’ll need to determine the underlying cause. Perform an initial visual check. If it’s algae, the water will often also appear cloudy, not just tinted green. You may observe slimy or dusty film on pool walls, floors, or steps. Because algae tends to grow in shaded areas, corners, or low-flow zones, inspect those areas closely. If it’s algae, the color will also lighten temporarily after brushing, which will create visible debris in the water.
If metals are the cause, the water will be observedly green, but clear, and can range from “tea-colored” to a deeper emerald green. You won’t observe a slippery or slimy feel on surfaces, and you may also see color appear suddenly after adding chlorine. You might also observe stains–from green to brown or black–on walls or ladders.
Next, to confirm your visual observations, you’ll need to perform a water test.
If it’s algae, indicators include elevated combined chlorine, high CYA levels, or Free Chlorine levels that drop quickly after adding.
To test for metals, you can use a digital pool water test kit like the LaMotte WaterLink Spin Touch, as it can test for Iron and Copper, or purchase a specific metal test kit to test for Iron, Copper, and Manganese.
If Algae is Present
General steps to kill and remove algae are:
If Metals Are the Issue
Sometimes, pools have both algae and metals, requiring a combination of shock treatment and sequestrants.
While algae and metals can be introduced to your pools from external contaminants, including water sources, maintaining consistent, healthy water conditions through stable routines, monitoring, and proactive maintenance is your best method of prevention.
Fortunately, Pool Shark H2O streamlines testing, improves accuracy, and reduces the risk of persistent issues like green pool water. The app automatically performs complex chemical equations for you, reducing human error and locking test and adjustment records. This makes it simple for staff and operators to reference records and identify issues early. It also recommends adjustments, acting as backup for your Certified Pool Operators (CPOs) and ensuring you and your staff take the appropriate, MAHC-aligned steps to remediate issues.
When used with the LaMotte WaterLink Spin Touch, you can perform water tests in less than a minute–and with advanced water parameter measurement, catch building metals or CYA issues early, before they turn your pool water green.
WHY IS MY POOL GREEN? FAQs
Can a pool turn green even if chlorine levels are normal?
Yes. High CYA can lock chlorine, reducing its effectiveness. Even with free chlorine in range, algae can survive if CYA, pH, or alkalinity are out of balance.
How can I tell if green water is caused by metals instead of algae?
Metal-contaminated water often appears green or brown but doesn’t form a film or attach to surfaces like algae. Test kits or lab analysis confirm copper, iron, or manganese presence.
How do I know how much shock to add for a green pool?
The correct shock dose depends on your pool’s water volume and CYA level. Large pools or pools with high CYA require higher chlorine levels to effectively kill algae. Use accurate measurements of volume and consider using a pool volume calculator or manufacturer guidelines to determine the proper dosage. Always follow chemical label instructions and ensure circulation while shocking.
How can I prevent my pool from turning green in the future?
Maintain consistent chlorine levels adjusted for CYA, regularly check pH and alkalinity, inspect water sources for metals, brush and vacuum surfaces, and ensure circulation reaches all areas. Documenting chemical trends helps anticipate issues.