Why is my pool still green after I shocked it?

Having a green pool can be frustrating, especially when you’ve tried shocking it but the water is still green. There are a few key reasons why your pool may remain green even after shocking.

Not Using Enough Shock

One of the most common reasons a pool stays green after shocking is not using a high enough dose of shock (chlorine). The amount of shock required depends on the size of your pool, the current chlorine levels, and how bad the algae growth is.

As a general rule of thumb, to shock a green pool you need to use 5-10 times the normal chlorine dose. So if you normally use 1 lb of chlorine per 10,000 gallons weekly, you may need to use 5-10 lbs per 10,000 gallons to properly shock the pool.

Test the chlorine level after shocking – it should be around 10 parts per million (ppm) to clear up the green water. If it’s below 3 ppm, you’ll need to add more shock.

Improperly Preparing the Pool for Shocking

Before adding shock, the pool should be properly prepped to ensure the chlorine works effectively. This includes:

  • Brushing and vacuuming the pool to remove as much algae as possible
  • Cleaning the filter and skimmer baskets to remove debris
  • Balancing the water chemistry (pH, alkalinity, calcium hardness)
  • Adding shock at dusk when the sun won’t burn it off quickly

Skipping any of these steps means the shock will not work as well or as quickly to kill off the algae.

Persistent Algae Type

If you’ve properly shocked the pool with high enough chlorine but it remains green, the type of algae may be resistant and difficult to eliminate.

Black algae and mustard algae are two stubborn species that can tolerate even very high chlorine levels. They have thick cell walls that shield them from the sanitizer.

For these resistant strains, it may take several rounds of heavy shocking over 2-3 weeks to fully clear them. You may need to supplement with an algaecide as well.

Poor Filtration and Circulation

No matter how much shock you use, if the water circulation is poor then it won’t disperse well and be effective. Make sure your pump and filter are running optimally.

Check that pool skimmers are clear of debris like leaves that can block proper flow. Clean out baskets, pipes and inlets if needed.

Run the pump longer (up to 24 hours) after shocking to keep water moving. Increased flow creates better mixing and allows the shock to reach all areas.

Ignoring Other Pool Care

While shocking is important for a green pool, it can’t solve the problem alone. You still need to practice regular water care maintenance.

After the initial heavy shock, continue adding chlorine to maintain a 1-3 ppm level. Check pH and alkalinity balances. Brush and skim the pool frequently. Backwash filters when pressure builds up.

Slacking on overall care will allow algae and contaminants to return quickly, rendering the shocking ineffective.

New Water Addition

Adding fresh water to your pool can also turn it green if the new water contains algae, bacteria or other contaminants. When refilling ensure the new water is balanced and sanitized.

Shock the entire volume of water after refilling – don’t just multiply your regular dose by the amount added.

Run filters and let the chemicals mix for a full day before testing or using the pool after new water is added.

Cold Weather

Colder temperatures can slow down the effects of shocking since chlorine works best in warmer conditions. It may take longer to see results shocking in cold water.

Make sure to test and adjust the pH even with cooler water temps. Low pH reduces chlorine’s effectiveness.

Consider using liquid chlorine or non-chlorine shock in winter as they work better in cold than powdered shocks.

Not Allowing Enough Time

After shocking a green pool, it takes some time for the chlorine to fully circulate and start killing algae. Overnight is usually minimum time needed before you’ll start seeing the water turn from green to clear.

Don’t expect instant results – shocking may take 1 to 5 days to fully clear a green pool depending on conditions. Be patient and allow proper contact time for the chlorine.

Insufficient Filtration

The filter system plays a crucial role in clearing up green pool water after shocking by removing dead algae, debris and other contaminants.

Make sure your filter can handle the size of your swimming pool. Undersized filters lead to slow turnover and poor filtration, taking longer to clear the water.

Frequently backwash and clean filters after shocking to prevent clogging and ensure proper flow. Increased run times (24 hours daily) are recommended.

Water Chemistry Issues

If the basic water chemistry is out of balance – pH, alkalinity, calcium hardness – shocking effectiveness is greatly reduced. Even with heavy doses of shock, an out of balance pool may stay green.

Test and adjust your chemicals to the ideal ranges. Correct any issues before and during the shocking process or you’ll just be wasting chlorine.

Misdiagnosing the Issue

Sometimes pool owners assume any type of discoloration or cloudiness must be algae related. But issues like a chlorine lock or water chemistry imbalance can also turn water greenish.

Shocking will provide little benefit in these cases. Properly diagnose the root cause of the green water for best results.

High Bather Load

The more people using your pool, the faster chlorine gets used up. With heavy bather loads from parties, pool events etc, shocking chlorine can be burned off before it has a chance to work on the algae.

Shock at night after guests leave for lower chlorine demand. Increase dosage and frequency when bather loads are high to keep chlorine at proper levels.

Nitrates In Source Water

The fill water coming into your pool often contains nutrients like phosphates and nitrates that can fuel algal growth. These are food sources for algae even in the presence of chlorine.

Use a specialty product designed to remove phosphates and nitrates from the water if they are above target levels. Reduce nutrients for clearer water.

Too Much Cyanuric Acid

Cyanuric acid (stabilizer) protects chlorine from UV rays, but too much can negatively impact its effectiveness. Per the CDC, cyanuric acid should be 30-50 ppm.

High cyanuric levels over 100 ppm hinder chlorine’s sanitizing power. Reduce stabilizer concentration through draining and refilling part of the pool.


Shocking a green pool with chlorine is usually effective when done properly, but there are some scenarios where the water remains discolored afterwards:

  • Insufficient chlorine dosage
  • Failure to prep pool before shocking
  • Stubborn, resistant algae types
  • Poor circulation and filtration
  • Imbalanced water chemistry
  • Cold water temperatures
  • Not allowing enough contact time
  • New water addition to pool
  • Overuse from heavy bather load
  • High nutrient levels
  • Excessive cyanuric acid concentration

Troubleshooting these issues and adjusting your shocking process accordingly will help clear up persistent green pool water so you can get back to swimming.

Related Questions

How much shock should I use for a green pool?

For a green pool, you generally need to shock with 5-10 times the normal chlorine dose. So if you normally use 1 lb per 10,000 gallons weekly, use 5-10 lbs per 10,000 gallons to clear up green water. Test chlorine after shocking to ensure it reaches 10 ppm.

Can I swim right after shocking my pool?

It’s best to wait at least 24 hours after shocking a green pool before swimming in it. This allows time for the chlorine to fully circulate and start clearing the algae. The initial super-chlorination levels after shocking are unsafe for swimmers.

What’s the fastest way to get rid of green pool water?

The fastest way is to shock the pool with 10 times the normal chlorine dose after thoroughly cleaning the filter and brushing walls. Run the pump 24 hours a day until water clears. Supplement with algaecide if needed for resistant strains.

Why is my pool still cloudy after shocking it?

A cloudy pool after shocking is typically due to debris and particles like dead algae floating around. Ensure filters are clean so they can capture contaminants. Increase run times to 24 hours and manually vacuum/brush the pool. Over time it will clear up.

Can you have clear water with no chlorine?

Having clear pool water with no chlorine is very difficult. Non-chlorine shock can temporarily sanitize water and oxidize organic matter. But without a regular sanitizer like chlorine, the pool water will quickly turn green and cloudy due to uncontrolled algae and bacteria growth.

Troubleshooting Green Pool by Issue

Issue Cause Solution
Insufficient chlorine Too low chlorine dosage; chlorine demand too high Shock at 10x normal dose; maintain 1-3 ppm FC
Improper prep Filter not cleaned; water not balanced; shocking during day Clean filter & baskets; balance water; shock at dusk
Stubborn algae Black algae or mustard algae Heavy shocking over 2-3 weeks; algaecide
Poor circulation Dirty skimmers, clogged intakes; dead pump Clean all baskets; check pump operation
Other care neglected No brushing, pH unchecked, infrequent filtering Regular entire care regimen
New water added Untreated new fill water unbalanced Balance and shock new water before adding
Cold temperatures Chlorine less effective in cold water Liquid or non-chlorine shock; adjust pH
Insufficient time Expecting instant results Wait 24+ hours after shocking before retesting
Small filter Filter too small for pool volume Upgrade to adequately sized filter
Imbalanced water Incorrect pH, alkalinity, or calcium hardness Test and correct water balance
Misdiagnosis Assuming algae caused green water Properly diagnose root cause first
Heavy bather load Chlorine burned off quickly Shock at night after guests leave
High nitrates Excess nitrates in fill water feeding algae Use nitrate remover product
Too much stabilizer Cyanuric acid over 100 ppm Drain and dilute cyanuric to 30-50 ppm

Preventing Green Pool Water

While shocking can clear up a green pool after the fact, prevention is always better. Here are some tips to avoid getting a green pool in the first place:

  • Maintain proper chlorine levels 1-3 ppm
  • Regularly test and adjust pH, alkalinity, etc
  • Shock pool weekly or as-needed
  • Brush pool walls and vacuum bottom regularly
  • Clean pump basket, skimmers and filters
  • Watch for algae early and treat quickly
  • Address misbalanced water right away
  • Shock after heavy rains or bather loads
  • Keep phosphates, nitrates and stabilizer in check
  • Don’t let leaves, debris accumulate

Staying on top of these maintenance tasks makes it much less likely your pool water will turn that dreaded green color during peak swimming season.

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