Why Is My Pool Still Green After Algaecide and Shock

What is the recommended free chlorine level for effectively killing persistent algae?

To eradicate a stubborn algae bloom, you must elevate free chlorine levels to a specific "breakpoint" relative to your cyanuric acid concentration. For most residential pools, this requires maintaining a free chlorine level between 10 and 20 parts per million. For instance, if your cyanuric acid is at 50 parts per million, you should aim for the higher end of that range to achieve the necessary oxidation potential to kill the spores.

The nuance here involves the total chlorine demand of the water. If your pool is heavily contaminated with organic debris, the chlorine will be consumed rapidly, dropping your levels back to zero within hours. In such cases, testing every four hours and re-dosing is mandatory. An exception exists for salt-chlorine generators, which cannot reach these high levels safely; you must supplement with liquid chlorine to shock the pool.

How does pH balance influence the chemical efficacy of pool shock?

The efficacy of hypochlorous acid—the active killing agent in chlorine—is directly tied to pH. At a pH of 7.2, roughly 65 percent of your chlorine is active. If the pH rises to 8.0, that active portion plummets to about 20 percent. Many people assume that adding more shock will compensate for a high pH, but this is counterproductive. You are simply wasting expensive chemicals because the alkalinity and high pH will continue to neutralize the chlorine's ability to sanitize the water, leaving the algae largely untouched.

Why might a high phosphate level prevent algaecide from working correctly?

Phosphates act as a primary nutrient source that fuels rapid algae growth, effectively creating a buffet that allows algae to reproduce faster than standard algaecides can kill it. When phosphate levels exceed 500 parts per billion, the chemical demand becomes unmanageable. If you treat a pool with high phosphates, the algaecide will bind to the nutrients and become inert before it ever touches the algae cells. You essentially end up with a pool that is chemically treated but biologically thriving.

A practical example is a pool surrounded by heavy landscaping or lawn fertilizer runoff, which introduces constant phosphate loads. The nuance is that while phosphate removers can clear the food source, they often cloud the water temporarily as they precipitate the minerals. You must be prepared to vacuum or filter out this resulting sediment to see true clarity.

What role does pool filtration play in clearing cloudy or green water after treatment?

The filtration system acts as the mechanical engine that physically extracts dead algae particles from the water column after the chemicals have neutralized them.

• Pressure gauges: A rise of 8 to 10 pounds per square inch above the clean starting pressure indicates the media is clogged with trapped debris.
• Flow rates: A noticeable reduction in the water velocity returning to the pool through the jets signals that the filter is struggling to cycle the volume.
• Turbidity levels: If the water remains hazy despite high chlorine levels, the filter is likely bypassing particles or is too saturated to capture smaller organic matter.
• Backwash cycles: Frequent discharge of water is required when the filter media reaches its capacity to hold sequestered contaminants.

How long should a pool pump run during the recovery process from an algae bloom?

Yes, the pump must run 24 hours a day during the entire recovery phase until the water is crystal clear. This continuous circulation makes sure that the shock reaches every corner of the pool and that the filter can capture the dead algae before it settles on the floor.

When is it necessary to use a flocculant instead of standard shock?

Flocculant is necessary when the water is clouded with microscopic particles that are too small for the filter to trap effectively.

• Cloudiness: When the water looks like skim milk and does not improve after 48 hours of constant filtration, a flocculant is the only way to clear it.
• Dead algae: After a successful shock treatment, a layer of fine, greyish dust often settles on the bottom, which a flocculant can clump into a vacuumable mass.
• Vacuuming: This process requires vacuuming to waste, as the heavy clumps will clog the filter media if you attempt to run them through the system.
• Settling time: You must turn off the pump for 8 to 12 hours after application to allow the particles to drop to the floor.

Can sunlight and UV exposure degrade chlorine levels before they eliminate algae?

Yes, ultraviolet rays from the sun can destroy up to 90 percent of unprotected chlorine in just two hours. This degradation is why stabilizers like cyanuric acid are used, but even with proper stabilization, you should shock your pool at dusk to maximize the time the chlorine has to work.

What are the signs that a pool filter needs backwashing or cleaning to restore circulation?

You should clean or backwash your filter when the internal pressure rises significantly above the baseline established after a fresh cleaning.

Compare your current gauge reading against the baseline you recorded when the filter was last serviced to determine if it is time to act.