Green Pool Water: Causes and Quick Solutions

Introduction

Did you wake up this morning to find your pool water completely green? This is one of the most common problems pool owners face, especially during the hot summer months in the Margem Sul region of Lisbon. Green pool water is not just an aesthetic issue – it represents a chemical imbalance that can make the water unsafe for swimming.

The good news is that a green pool can be completely recovered within 24 to 48 hours with the correct procedures. In this comprehensive guide, we will explain the exact causes of green water, the step-by-step treatment to recover your pool, and preventive measures to ensure it doesn't happen again.

If you live in areas like Almada, Seixal, Setúbal, or anywhere in the Margem Sul, understanding the particularities of our climate is essential for effectively preventing and treating this problem.

Why Does Pool Water Turn Green?

The green colour in pool water is caused by microscopic algae that multiply rapidly when they find favourable conditions. These algae don't appear from nowhere – the spores are present in the air, in rain, and can be carried by the wind. When they find the right environment, they explode in exponential growth.

Main Causes of Green Water

The most common causes for algae proliferation are:

• Insufficient chlorine levels: When free chlorine falls below 1 ppm, algae begin to develop
• Unbalanced pH: A pH above 7.6 drastically reduces chlorine effectiveness
• Inadequate filtration: Pump running too few hours or dirty filter
• High temperatures: Above 27°C, algae multiply much faster
• Lack of preventive algaecide: Especially important during summer
• After storms: Rain brings spores, dust, and alters chemical balance

In Margem Sul, the high summer temperatures combined with periods of strong wind (which bring dust and spores) create ideal conditions for algae growth.

Types of Algae and How to Identify Them

There are three main types of algae that affect pools:

Green algae are the most common and fortunately the easiest to eliminate. If your pool has uniformly green water, this guide will help you solve the problem.

How to Recover a Green Pool: Step-by-Step Guide

Before starting treatment, gather the necessary materials:

• Water test kit (pH, chlorine, alkalinity)
• Shock chlorine (calcium hypochlorite or dichloroisocyanurate)
• pH regulator (if needed)
• Shock algaecide
• Surface net
• Pool vacuum
• Wall brush

Step 1: Test the Water

Start by testing the water parameters:

• pH: Should be between 7.2 and 7.4 (ideal for shock treatment)
• Free chlorine: Probably at 0 or very low
• Alkalinity: Should be between 80-120 ppm

If pH is above 7.6, add pH reducer before shock treatment. Chlorine is much more effective with the correct pH.

Step 2: Physical Removal of Debris

Before adding chemicals:

1. Use the net to remove leaves, insects, and large debris
2. Brush walls, stairs, and corners vigorously
3. Don't vacuum yet – you'll pick up dead algae later

This step is crucial because algae adhere to surfaces and chemicals alone won't completely remove them.

Step 3: Shock Treatment

Shock treatment consists of drastically raising chlorine levels to kill algae. The dose depends on severity:

• Slightly green water: 2x normal shock dose
• Medium green water: 3x normal dose
• Very green/opaque water: 4x normal dose

For a 50m³ pool with very green water, you may need 1 to 1.5 kg of shock chlorine.

Step 4: Keep the Pump Running

After adding shock chlorine:

• Keep the pump running 24 hours non-stop
• This ensures chlorine distributes evenly
• Check the filter every 6-8 hours and backwash if pressure rises

Step 5: Add Algaecide

4 to 6 hours after shocking, add shock algaecide following manufacturer's instructions. The algaecide kills surviving algae and prevents regrowth.

Step 6: Wait and Observe

In the first 12-24 hours, the water will go through several phases:

1. Dark green to light green: Algae are dying
2. Light green to cloudy/milky: Dead algae in suspension
3. Cloudy to clearing: Filtration is removing residue

If after 24 hours the water is still green, repeat the shock treatment.

Step 7: Vacuuming and Final Cleaning

When water starts clearing:

1. Carefully vacuum the bottom (waste mode if possible, to avoid recirculating)
2. Perform complete filter backwash
3. Test water again and balance pH
4. Add preventive algaecide

How Long Does It Take to Recover a Green Pool?

Recovery time depends on severity:

• Slightly green water: 24-48 hours
• Moderately green water: 48-72 hours
• Very green/opaque water: 3-5 days

Factors affecting recovery time include the filtration system capacity, water temperature, and whether treatment was done correctly from the start.

Common Mistakes to Avoid

Many owners make mistakes that prolong or worsen the problem:

1. Insufficient chlorine dose: Better to overdo it than underdo it
2. Not adjusting pH first: Chlorine is ineffective with high pH
3. Turning off pump at night: It needs to run continuously
4. Vacuuming too early: Wait until water clears
5. Not brushing walls: Algae adhere to surfaces
6. Using algaecide only: Algaecide alone won't solve it, you need chlorine

Prevention: How to Keep Your Pool From Turning Green

Prevention is always easier and cheaper than treatment. Follow these recommendations:

Regular Maintenance

• Test water 2-3 times per week in summer
• Maintain chlorine between 1-3 ppm
• Maintain pH between 7.2-7.6
• Backwash filter weekly
• Use preventive algaecide weekly

Special Care

• After storms, do a preventive shock
• With temperatures above 30°C, increase testing frequency
• During periods of heavy use, reinforce chlorine
• Keep skimmers and baskets clean

For Margem Sul Residents

In our region, some additional care is important:

• Coastal zones (Costa da Caparica, Sesimbra): Wind brings more spores and dust
• Rural zones (Palmela, Azeitão): Pollen and leaves can clog filters quickly
• Well water (Moita, Montijo): Iron in water can cause additional problems

When to Call a Professional

Although most green water situations can be resolved by the owner, there are cases when it's better to contact a specialist:

• Water remains green after 3 shock treatments
• Suspected black algae (dark spots that won't come off)
• Problems with filtration equipment
• You don't have time or availability for treatment
• Pool has been abandoned for a long time

Treatment Costs

The cost of treating a green pool varies according to severity:

These values include shock chlorine, algaecide, and water tests. The cost of a complete professional service can range between €100-200, but includes guaranteed results.

Frequently Asked Questions

Is green water dangerous for health?

Yes, it can be. The algae themselves are not all dangerous, but green water indicates that disinfectant levels are low. This allows the development of harmful bacteria like E. coli and Pseudomonas. Swimming in a green pool is not recommended.

Can I use household bleach to treat the pool?

Technically yes, but it's not recommended. Household bleach has only 2-5% sodium hypochlorite, while pool products have 10-15%. Additionally, it may contain perfumes and other harmful additives. Always use pool-specific products.

How long after shock treatment can I swim?

Wait until chlorine level drops below 3 ppm (ideally 1-3 ppm). This usually takes 24-48 hours. Always test before swimming.

Can green water damage the pump or filter?

Indirectly, yes. Algae can clog the filter and increase pressure, straining the pump. That's why it's important to backwash frequently during treatment.

Why does my pool turn green every year?

There's probably an underlying issue not resolved: insufficient filtration capacity, inadequate winter treatment coverage, or the dosing system isn't optimised. A professional evaluation can identify the root cause.

Conclusion

Recovering a green pool is perfectly possible within 24 to 48 hours with the correct procedure. Key points to remember are:

• Adjust pH first before shock treatment
• Use sufficient chlorine dose – don't skimp on chemicals
• Keep pump running 24 hours during treatment
• Brush the walls to dislodge algae
• Prevention is better than cure – regular maintenance is fundamental

If you need professional help with your pool in Margem Sul, ManutençãoPiscinas is here to help. Contact us for a no-obligation quote.