Green water: why it happens and how to clear it

What is happening in the water

Algae arrive every day with wind, rain and leaves. As long as there is enough free chlorine, they don't take hold. Water turns green when that chlorine runs out or stops working, and in Costa Rica there are three typical moments: after several days of downpours, which dilute the chlorine and wash in soil and leaves; during weeks of strong sun and heat, which use it up faster; and when the pump sits off or the filter is dirty and the water doesn't circulate.

Not all green is algae

If the water is green but transparent — you can see the bottom sharply — and it changed colour right after adding chlorine, the likely cause is dissolved metals, such as copper or iron, reacting with the chlorine. More chlorine won't fix that: it calls for a metal sequestrant and finding out where the metals come from. If it is green and cloudy, it is algae, and the rest of this guide is for that case.

How to clear it, in order

1. 1

   Test before adding anything

   Free chlorine and pH, with a DPD drop kit: it is the method the CDC recommends as the most accurate. Write the numbers down; they are your starting point.

2. 2

   Remove debris and brush

   Leaves and debris, out. Then brush walls, floor, steps and corners: algae cling to the surface and chlorine can't reach them unless they are loosened.

3. 3

   Clean the filter

   Backwash the sand filter or rinse the cartridge. A saturated filter gives back to the pool whatever it takes out.

4. 4

   Bring the pH into range

   With a high pH, the chlorine you add won't do much. Correct it first.

5. 5

   Shock the pool

   Apply shock chlorine following the product label and your pool's volume, preferably at dusk so the sun doesn't burn it off. Never mix chemicals together, and always add the product to water, not the other way round.

6. 6

   Filter non-stop

   Leave the pump running continuously until the water clears. It usually goes from green to a cloudy blue-grey: that is dead algae, and a good sign.

7. 7

   Vacuum and test again

   Vacuum what has settled, clean the filter again and test. You can get back in when the water is clear, you can see the bottom, and chlorine and pH are back in range.

How to keep it from coming back

• Test chlorine and pH every day the pool is used — the CDC recommends at least twice a day — and always after a heavy downpour.
• Run the filter long enough every day to turn over all the water.
• Brush once a week, even if it looks clean.
• Keep the baskets and the filter clean.

When to call a technician

If the green comes back every few weeks, if the water won't clear after a properly done treatment, or if you suspect metals or a filter that no longer filters, there is an underlying problem, and adding more chemical gets expensive. At Jireh we provide monthly maintenance with CPO-certified technicians: scheduled visits, water balancing with chemicals included, vacuuming and brushing, pump and filter checks, and a log of every visit. We also service pools we didn't build.