Green water is one of the most common surprises a spa owner gets, and it always feels sudden. One evening the water is clear, and by the weekend it has a green tint and a slick feel. The cause is almost always the same, and so is the fix.
What green water actually is
Green water is algae. Algae spores are always in the air and always landing on the surface of your spa. The only thing keeping them from blooming is your sanitizer. When the sanitizer level drops, even for a day or two, the spores take hold and multiply. Warm water speeds this up, so a hot tub can go from clear to green over a single weekend.
The green tint is the visible stage. By the time you can see it, the bloom is already established and the water is no longer safe to sit in.
Why it happens so fast
Sanitizer gets used up. Every bather, every bit of debris, and every hour of sunlight burns through it. If you top it up by hand a few times a week, you are guessing at a level that changes by the hour. A quiet week, a warm spell, or a busy weekend can pull the sanitizer down to nothing while you are not looking.
The number that matters is not how much sanitizer you added. It is how strongly the water is actually killing bacteria right now, measured as ORP in millivolts. Hold ORP between 650 and 750 mV and algae cannot get a foothold. Let it fall below that and the door is open.
The pH connection
Sanitizer only works well inside a narrow pH window. If pH climbs above 7.6, even a decent dose of chlorine loses most of its killing power. So high pH and low sanitizer often arrive together, and the water turns green faster than the sanitizer reading alone would suggest. Keep pH between 7.2 and 7.6 and your sanitizer earns its keep. Our guide to water balance explains why this window matters so much.
How to clear green water
Step 1: shock the water
Add a large dose of sanitizer, called a shock, to overwhelm the algae. Follow the dose on the product for your water volume. The water may turn cloudy or grey as the algae dies. That is normal.
Step 2: get pH in range
Check pH and bring it to between 7.2 and 7.6. If pH is high, the shock will not do its job. Add a pH decreaser if you need to.
Step 3: run the pump and filter
Circulate the water so the filter can catch the dead algae. Rinse or clean the filter as it loads up. This can take several hours.
Step 4: hold the sanitizer in range
Once the water clears, the job is to keep ORP between 650 and 750 mV so it never happens again. This is the part people get wrong, because holding a level by hand means testing constantly.
Stop it coming back
The reason green water is a repeat problem is that nobody is watching the sanitizer between tests. E.W.A. reads pH, sanitizer and temperature every ten minutes, the same way every time, and tells you in plain words the exact dose to add before a problem starts. Instead of finding green water on Saturday, you get a clear instruction on Tuesday.
When you are ready to stop guessing at your water, get E.W.A. and let it watch the numbers that turn a spa green.