How to Clear Pool Water After an Algae Bloom

An algae bloom can turn your pool from clear to green soup overnight. But with the right plan, patience, and pool care, you can bring it back to life without draining everything. Here’s how to do it right.

What Causes Algae Blooms?

Algae blooms usually form because of poor water circulation, imbalanced chemicals, or neglect. Here are the usual suspects:

Low chlorine or sanitizer levels
Warm weather with stagnant water
Clogged filters or weak water flow
Excess phosphates and organic debris

Once algae gets a foothold, it multiplies quickly, and the water becomes cloudy, green, or even black depending on the type.

Why It’s Important to Act Fast

Waiting even a day or two can make the recovery harder. Algae weakens your chlorine’s ability to sanitize, clogs up filters, and stains pool surfaces. If left untreated, it can damage equipment and make the pool unsafe.

Step 1: Test and Balance the Water

Before throwing in chemicals, test the following levels:

Free chlorine: Should be near zero during a bloom
pH: Aim for 7.2 for the shock to work effectively
Alkalinity: Between 80 and 120 ppm
CYA (stabilizer): If it’s too high, the chlorine may be ineffective

Lower your pH first if it’s above 7.4. This helps your chlorine shock work better.

Step 2: Brush the Pool Thoroughly

Use a stiff pool brush to scrub:

Walls
Floor
Steps
Behind ladders

This removes algae from surfaces so the shock treatment can kill it completely. Focus especially on corners where algae hide.

Why Brushing Is Essential

Even the best shock treatment cannot kill algae that is stuck to pool surfaces. Brushing breaks apart biofilms that protect algae, exposing it to chlorine and speeding up the cleanup.

Step 3: Shock the Pool

You need a heavy dose of chlorine to break through the bloom.

Use calcium hypochlorite or liquid chlorine
Follow instructions based on pool size, but most blooms need at least 3 times the normal shock dose
Distribute it evenly around the perimeter
Run the pump continuously

Helpful Tip:

Shock the pool at night to prevent sunlight from breaking down the chlorine too quickly.

What Happens After Shocking?

You may notice the water turns gray or blue as algae dies. Do not panic. This is a good sign. Your filter will handle the rest.

Step 4: Run the Filter Nonstop

Keep the filter running 24/7 for at least the first 48 hours. During this time:

Backwash your sand or DE filter frequently
Rinse cartridge filters when pressure rises
Consider using a clarifier or flocculant to speed up debris removal

Additional Filtration Tips:

Clean filters thoroughly at the end of the process
If using flocculant, make sure your pool can vacuum to waste, as floc binds particles to sink them

Step 5: Vacuum the Pool

After a day or two of filtering:

Manually vacuum debris to waste if your filter allows
If not, vacuum slowly and clean filters regularly
Do not use automatic pool cleaners, which can stir up debris

Manual vs. Automatic Vacuuming

Manual vacuums are best after an algae bloom because they remove debris without disturbing it. Automatic vacuums may just push particles back into the water.

Step 6: Retest and Adjust

Once the water is clearer:

Retest pH, chlorine, and alkalinity
Rebalance everything to ideal levels
Add algaecide as a preventative, not a treatment

Important Reminder:

Wait until chlorine levels are below 5 ppm before adding algaecide. High chlorine can destroy the effectiveness of the product.

Step 7: Keep a Prevention Routine

Now that you survived an algae bloom, avoid the next one with routine care:

Brush weekly
Vacuum regularly
Shock after heavy use or rain
Maintain chlorine between 1 to 3 ppm
Check filter pressure and clean as needed

Weekly Pool Checklist:

Test water chemistry
Brush pool walls and floor
Empty skimmer baskets
Run the pump at least 8 hours
Check filter pressure and backwash

When to Call in a Pro

If your algae bloom is severe and your water is dark green or black, or if your efforts are not improving the clarity after a few days, call a professional. You might be dealing with black or mustard algae, which are harder to remove.

Professional help may also be needed if:

The pool is large or used heavily
Equipment is outdated or underpowered
You’re unsure how to safely adjust chemicals

Clearing an algae bloom is not fun, but it is doable. Stay consistent, stay patient, and your pool will return to its crystal-clear state.