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Why Is My Pool Green?

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Few things panic a pool owner faster than pulling back the cover and seeing green water staring back at them. It looks dramatic, feels urgent, and raises the same question every time:

“What went wrong?”

The short answer: your pool didn’t suddenly turn green.
The long answer is more useful — and fixable.

Green Pool ≠ One Problem

A green pool is almost always the result of several things lining up at once. Algae is opportunistic — it doesn’t need much encouragement.

Here are the most common reasons your pool turns green:

1. Not Enough Sanitizer

Chlorine (or bromine, in spas) is what keeps algae from growing.
If sanitizer levels drop too low — even briefly — algae can take hold.

This often happens:

  • After heavy rain
  • During heat waves
  • When the pool is used more than usual
  • When chlorine demand increases but dosing doesn’t

Important:
You can have chlorine in the water and still not have effective chlorine. Which brings us to…

2. Poor Water Balance

Sanitizer works best within a narrow chemical window. If pH, alkalinity, or stabilizer (CYA) are out of range, chlorine becomes less effective — sometimes dramatically so.

Common scenarios:

  • High pH → chlorine becomes sluggish
  • Excess stabilizer → chlorine is “locked up”
  • Low alkalinity → chemistry becomes unstable and unpredictable

The water may test fine at a glance but still be primed for algae.

3. Inadequate Circulation or Filtration

Algae loves still water.

If your pump isn’t running long enough, your filter is dirty, or certain areas of the pool don’t circulate well, algae can establish itself even when chemistry looks decent.

Watch for:

  • Dead spots on steps or ledges
  • Poor skimmer action
  • Pressure gauges you’ve learned to ignore

Clear water doesn’t always mean clean water.

4. Organic Contamination

Leaves, pollen, sunscreen, body oils, and debris all consume chlorine.

After storms, pool parties, or long periods without maintenance, sanitizer demand spikes — and algae takes advantage of the gap.

This is why pools often turn green:

  • Right after opening
  • After vacations
  • After heavy winds or rain

5. “I Shocked It — Why Is It Still Green?”

Shocking a pool is not a magic spell.

If:

  • The water is out of balance
  • The filter can’t keep up
  • The algae load is already established

…then shock alone won’t fix it. In some cases, it can even make things worse by clouding the water without fully killing the algae.

So… How Do You Fix a Green Pool?

There’s no single universal recipe — and that’s the point.

A proper fix requires:

  1. Accurate water testing
  2. Correcting balance first
  3. Applying the right sanitizer strategy
  4. Supporting the process with filtration and circulation

Skipping steps leads to frustration, wasted chemicals, and repeat problems.

The Takeaway

A green pool isn’t a failure — it’s a signal.

It’s your pool telling you that:

  • Chemistry is out of sync
  • Sanitizer demand exceeded supply
  • Or circulation isn’t doing its job

The good news?
Green pools are almost always recoverable — when treated correctly.

If you’re guessing, you’re gambling.
If you’re testing properly, you’re in control.

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