Green water is algae, and algae grows when the chlorine ran short for long enough.
What let it grow
- The chlorine ran short for your stabilizer level. Stabilizer shields chlorine from the sun but also slows it down, so the more stabilizer in the water, the more chlorine it takes to keep algae out. A level that was enough last month can be too low now. The app sets your chlorine target from your stabilizer reading.
- Heat, sun and a busy week. Warm water grows algae faster and strong sun burns chlorine off faster, so a hot spell, a crowded weekend or a heavy rain can use up the chlorine sooner than you expect.
- The water was not moving enough. Algae starts where the water sits still: corners, steps and the shade behind a ladder. Short pump hours or a filter that needs cleaning give it those spots.
- Stabilizer has built up past what chlorine can keep up with. Every chlorine tab adds stabilizer, and stabilizer does not leave on its own. Past a point no safe amount of chlorine gets ahead of it, and the answer is replacing some of the water, which the app sizes for you in gallons.
Hazy green, green, or worse
When you log a test in the app, the question "How does the water look?" offers Hazy green, Green, Mustard and Black. Pick the one closest to what you see. Any of the four starts a clearing plan instead of a routine top-up, because algae you can see needs a chlorine level held for days rather than a single dose. Cloudy water with the chlorine gone is treated as early algae too, since that is usually what it is.
Before you buy anything
Test the water first. A green pool needs chlorine, time, a brush and the filter, and the products shelved beside them do not clear an established bloom: algaecide slows algae in water that is already balanced, and a clarifier only helps the filter at the very end. How the clearing works, step by step, is below.
Keep everyone out until it is finished
Do not swim in green water. You cannot see the bottom, and water with algae growing in it is water the chlorine is not keeping clean. The pool is ready again when the overnight test says the algae is gone and the chlorine has come back down to its everyday range, not when the water first looks clear.
The short versionGreen means the chlorine ran short long enough for algae to take hold. Test, let the app set the level for your stabilizer, and hold it until the overnight test says it is over.
How to clear a green pool
A green pool is cleared by a level you hold, not by a dose you add.
A green pool has an established algae bloom, and it is the one situation the normal weekly routine cannot reach. Clearing it takes a few days and it works reliably when you do all four parts — a raised chlorine level held continuously, brushing, filtration, and one test at the end that tells you it is over. Nothing about a green pool means you did something wrong; a warm week and a few days of low chlorine is all it takes.
Why one dose cannot do it
Algae consumes chlorine. Pour in enough to reach a high reading and a bloom will eat through it within hours, so the water is green again in the morning and the number is back where it started. That is not a failed dose, it is the bloom feeding. Clearing a pool means holding the chlorine at a raised level continuously until there is nothing left to consume it, which is a different job from correcting a reading.
What a sustained hold means in practice
You raise the chlorine to a specific level, then test and top it back up to that same level several times a day, for as many days as it takes. The level is not a fixed number — it is set by your stabilizer reading, and the app works it out and sizes each top-up. What matters is that the water is never left to fall back down between doses, because every hour it spends low gives the bloom time to recover.
Brush every surface, every day
Algae grips walls, steps and the shaded corners behind ladders as a film that chlorine struggles to penetrate. Brushing breaks the film and puts the algae into the water where the chlorine can reach it, and it lifts settled debris for the filter. Brush the whole pool once a day while you are clearing, and give the steps, the corners and anywhere the sun does not reach the most attention.
Run the filter around the clock
Chlorine kills the algae and the filter removes it, so both have to be working. Run the pump continuously for the whole process and check the pressure daily, because a filter working through a bloom loads up fast and a clogged filter stops the water clearing no matter how good the chemistry is. Clean or backwash it whenever the pressure climbs well above its clean reading.
The overnight test is the finish line
Clear water is not the same as finished water. The test that tells you the bloom is dead is a comparison: record the chlorine at dusk, keep the pump running overnight, and read it again before the sun comes up. If the level has barely moved, nothing living is left in the water eating chlorine, and you are done. If it dropped, the bloom is still alive and you hold the level for another day. Running it overnight takes the sun out of the equation, so anything that consumed the chlorine was something alive.
Why algaecide is not the fix
Algaecide is sold as the green-pool product and it is the one thing on that shelf that will not clear a green pool. It is built to slow algae down in water that is already balanced, not to remove an established bloom, and it does nothing about the organic load a bloom leaves behind. The copper-based kinds can stain a plaster surface permanently, and the rest tend to foam. Held chlorine is what clears the water, and a clarifier can help the filter catch the dead algae once the chlorine has finished.
The short versionHold the level, brush, filter, and let the overnight comparison tell you when it is over. It takes days rather than one trip to the store, and it works.
Every level on this page is set by your own readings, which is why none is written here. Put your test through the free calculator for the amount for your pool, or read why a pool goes cloudy if it is hazy rather than green.
