Guides / How to fix green pool water, step by step
How to fix green pool water, step by step
Green water means algae. The reliable fix is to hold a high chlorine level until the water is clear and stays clear overnight. Here is the step-by-step version, often called a SLAM (shock level and maintain) in pool communities.
Before you start
- You need a test kit that reads free chlorine and cyanuric acid (CYA). Strips are not accurate enough for this.
- Know your pool volume (calculator) and what chlorine you have (dose calculator).
- Remove leaves and debris. Pool covers and toys out.
Steps
- Test free chlorine and CYA. The shock level depends on CYA: about 40 percent of CYA (for example 16 ppm at 40 ppm CYA).
- Raise free chlorine to the shock level. Liquid chlorine works well. Run the pump the whole time. Add in steps and test.
- Brush the pool, walls and floor, to loosen algae and let chlorine reach it. Brush several times a day.
- Run the filter around the clock. Clean or backwash it when the pressure rises. Cloudy green water can clog a filter fast.
- Keep free chlorine at the shock level. Test every few hours and top up. Chlorine drops fast while algae are being killed.
- Do not add other chemicals yet, except what is needed to hold pH in range.
- Check for the end. The water is clear and: (a) the chlorine loss overnight is small (free chlorine drops by about 1 ppm or less from evening to morning), and (b) combined chlorine is under 0.5 ppm.
- Return to normal. Let free chlorine fall to your usual target, clean the filter once more, and retest everything.
How long does it take?
A light green pool can clear in two to three days. A dark green or black pool can take a week or more, mostly because of filtering. Time is not what ends it; the test at the end does.
If the water will not clear
- Is CYA above about 80 ppm? Chlorine is much slower. Plan partial draining.
- Phosphates or a failing filter or pump? Check circulation and filter condition.
- Run the pump and filter longer, and keep brushing. The filter is usually the bottleneck.
Want a step-by-step plan from your own test?
Clearly turns your pool or hot tub water test into an ordered plan: what to add, how much and in what order, for any brand of chemicals. It works offline and needs no account.
Clearly by Pulad Labs (Istanbul). Android, available on Google Play.
General information, not professional advice. Always follow the product label and your pool manufacturer's guidance. Never mix chemicals, add chemical to water (not water to chemical), wear eye and skin protection, and keep children and pets away while treating the water.
Frequently asked questions
How do I get rid of green pool water?
Raise free chlorine to the shock level for your cyanuric acid, brush, run the filter continuously and keep the level until the water is clear and the overnight chlorine loss is small.
How long does it take to clear a green pool?
A light green pool often clears in two to three days; a dark green or black pool can take a week or more.
Why is my pool still green after shocking?
Common reasons: too little chlorine for your CYA, high CYA slowing chlorine, a dirty filter, or not enough brushing and circulation.