Pool Care

How to Clear a Green Pool — Causes, Treatment and What to Expect

By Powell Pools  |  Pool & Hot Tub Maintenance, Buckinghamshire

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A green pool is one of the most common problems pool owners face — and it can happen at any point during the season, not just at opening time. A well closed down pool should come back clear in spring. But a sudden warm spell mid-season, a prolonged period of heavy rain, or simply falling behind on chemical checks can tip the balance quickly and leave you with green water seemingly overnight.

The good news is that with the right treatment approach it can be resolved. The less good news is that it takes time, the right products, and in many cases two visits to do it properly.

Here’s what causes a pool to turn green, how the treatment process works, and what you should realistically expect.

Why has my pool turned green?

Green water is almost always caused by algae — specifically algae that has been allowed to grow unchecked due to a breakdown in the pool’s chemical balance. This usually happens for one of a few reasons:

• Chlorine levels have dropped too low, removing the sanitiser that keeps algae in check
• pH has drifted out of range, making whatever chlorine is present less effective
• A sudden warm spell has accelerated algae growth faster than the existing chemical levels can handle
• Heavy or prolonged rain has diluted the pool water and thrown the chemical balance off
• Regular chemical checks have been missed — even a week or two without testing can be enough in warm weather
• The pool has been left over winter without adequate treatment or circulation
• A filtration or circulation problem has allowed water to stagnate

The darker the green, the longer the algae has been growing and the more established it will be. A light green pool that’s just starting to turn is much quicker to treat than one that’s been sitting dark green for several weeks.

The treatment process

Clearing a green pool properly takes more than just adding chlorine and hoping for the best. Every green pool is different, and the right treatment depends entirely on what the water test shows — which is why testing always comes first.

Sometimes the fix is straightforward — pH has drifted too high and is preventing the existing chlorine from working effectively. Correcting the pH can be enough to allow the sanitiser to do its job and the pool to clear on its own within a few days. In other cases chlorine has simply been depleted and the pool needs a significant dose to get back on top of the algae growth.

For more established algae blooms, a shock treatment is used — a large dose of chlorine designed to kill the algae rapidly. In some cases a flocculant may also be added, which binds suspended particles together and causes them to sink to the bottom rather than staying in the water and keeping it cloudy. Whether flocculant is used depends on the condition of the pool and what will give the best result — it’s not always necessary.

The pool is then run on recirculation for between one and four hours depending on the severity — this distributes the treatment evenly through the water. The filter is then turned off completely and left anywhere from 8 to 48 hours depending on how bad the algae bloom is. Once the water has cleared and the debris has settled on the bottom, it’s ready to vacuum to waste. Every pool is different and there’s no fixed timescale — it’s ready when it’s ready.

Why two visits are always needed

This is the part that catches people out. Once the flocculant has done its job and the dead algae has settled on the bottom of the pool, it needs to be vacuumed to waste — meaning it’s drawn out of the pool entirely rather than being pushed back through the filter.

This cannot be done on the same day as the initial treatment. The flocculant needs time to settle, and vacuuming too early will just stir everything back up and undo the work. A return visit is always required — typically 48 to 72 hours after the first.

Where flocculant has been used, a return visit is always required to vacuum the settled debris to waste — this can’t be done on the same day as the initial treatment. But even without flocculant, a follow up visit is recommended to check the water chemistry, top up chemicals if required, and confirm the pool is properly balanced before it’s handed back.

How long does it take to clear?

For a lightly green pool in reasonable condition, you can expect to see clear water within 4 to 7 days of the initial treatment. A heavily green pool that has been neglected for an extended period may take longer, and in some cases a second round of shock and flocculant is needed before the vacuum visit.

Water temperature also plays a role — warmer water responds faster to treatment than cold spring water, which is why early season green pools can sometimes take a little longer than expected.

Can I treat a green pool myself?

It’s possible, but the results are often inconsistent. Getting the chemical balance right before shocking, using the correct flocculant dose for your pool volume, and vacuuming to waste correctly all require a degree of experience and the right equipment. Done incorrectly, you can bleach a liner, damage a filter, or spend money on chemicals that don’t achieve anything.

If you’re not confident or simply don’t want the hassle, having it done professionally is usually the more cost-effective option when you factor in the chemicals, time and risk of getting it wrong.

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