Green, cloudy water with slippery walls or a slippery floor usually means algae have grown in the pool. If the water is still transparent but changes colour soon after chlorine is added, particularly after filling from a well, iron or other metals may be responsible. It is important to tell the difference because the treatments are not the same.
Written and edited by: Pool Explained editorial team
Check whether algae are the cause
Check whether you can clearly see the floor at the deepest point. If you cannot, keep everyone out until the water clears. Cloudiness is more than a cosmetic problem: it can hide someone who needs help.
Check a step or wall in a poorly circulated area for a slippery green or dark coating. This is a strong sign of algae. Growth often starts in corners, on steps, behind ladders and wherever circulation is weak. The water is often cloudy as well as green.
Think about the previous few days. Hot weather, heavy swimming use, low chlorine, a dirty filter or reduced pump hours can allow algae to grow. If the colour developed gradually and the walls became slippery, algae are more likely than metals.
Clear water that changes colour within hours of topping up or adding chlorine is different. This often happens with iron-bearing well water. Chlorine oxidises the dissolved metal, making it visible. The water may look greenish, yellow or brown while the floor remains easy to see.
How to treat algae
Test pH and free chlorine first. Do not immediately add shock treatment just because the water is green. High pH makes chlorine less effective, so first bring it within the range specified for your treatment. In outdoor pools, check stabiliser too: an excessive level can reduce chlorine's effectiveness.
Remove leaves and large debris, empty the skimmer and pump baskets, then check return flow and filter pressure. Brush the walls, steps and corners thoroughly. This loosens growth so chlorine can act on it and the filter can remove the residue.
Calculate the dose from pool volume, test results and the product's stated concentration. Products differ in strength, so there is no universal scoop size. Never mix chlorine with acid, pH products or a different chlorine type. Keep swimmers out during treatment.
Run filtration as the treatment instructions require and monitor filter pressure. Once algae die, the water may turn grey or milky. Those particles still need to be removed. Clean the filter when pressure rises and vacuum settled debris to waste if your installation supports it. Test again before deciding whether another treatment is needed.
What to do when the water is clear but coloured
If you can clearly see the bottom and a green or brown colour appeared just after adding chlorine, do not automatically add more. Further oxidation can intensify the colour and stain the liner, tiles or other finish. Record what you added and how much water you topped up immediately before the change.
Test both the fill water and the pool water. With well water, the usual checks include iron, manganese and copper, alongside pH and alkalinity. Strips can provide an initial indication, but persistent problems need a reliable sample analysis. Do not drain the entire pool before identifying the cause; a full water replacement is often unnecessary.
If metals are confirmed, use a treatment suited to the metal, its concentration and the pool material. Metal sequestrants can help prevent deposits and staining, but they do not necessarily remove metals from the water. Follow the dosing instructions; repeat additions may be needed. In some cases, partial water replacement or treatment of incoming water is appropriate.
Do not attack stains with random acids or strong cleaners. A treatment suitable for tiles may damage a liner or coating. If the metal is unidentified or the finish is sensitive, have the water analysed and check any proposed treatment on a small, inconspicuous area in accordance with the finish manufacturer's advice.
How to prevent green water returning
After treatment, note pH, free chlorine, water temperature, pump hours and what you added. A few notes on your phone are enough. They can help you see whether the problem follows hot weather, heavy use, well-water top-ups or reduced filter flow.
Regular tests, adequate free chlorine, the correct pH, a clean filter and good circulation are the main ways to prevent algae. Brush corners, steps and other places where dirt collects each week. An algaecide can help in some routines, but it does not replace chlorine or regular maintenance.
If you use well water, address metals before or during refilling rather than waiting until the whole pool changes colour. A source-water analysis shows what needs treatment. An automatic top-up can introduce small quantities repeatedly, allowing metals to accumulate and stain surfaces even when a single refill causes no obvious colour change.
Resume swimming only when the water is clear, the bottom is fully visible, test results are acceptable and the product's waiting time has passed. Clear-looking water is not automatically ready for swimming. Retest chlorine and pH before anyone gets in.