The short answer
Clear a green pool by lowering pH toward 7.2, then shocking with chlorine: raise free chlorine to roughly 40% of the cyanuric acid reading and hold it there, brushing and running the filter around the clock. Most blooms clear in 2-5 days. Retest often, re-dose to hold the level, and stop only when the water holds chlorine overnight.
A green pool is not a mystery and it is not a lost cause - it is an algae bloom, and algae dies when you hold free chlorine high enough, long enough, in water that lets the chlorine work. The mistake that turns a two-day job into a week is treating it as one big dose of shock instead of a sustained kill. Shock, walk away, and come back to a pool that is still green because the chlorine was eaten within hours and the bloom never finished dying.
For a service company, a green pool is a one-time recovery job that sits outside the normal weekly rate, and how you read it up front decides whether it costs you a couple of gallons of chlorine or a Saturday. Here is the exact chemical sequence, how to match the dose to how green the water is, why pH has to come down before you shock, why pools turn green in the first place, when chlorine alone will not cut it, and how to keep a whole route out of the green.
At a glance
Key takeaways
- Green water is an algae bloom from low free chlorine - clear it by shocking with chlorine, not by adding an algaecide as the main fix.
- Drop pH to about 7.2 before shocking; chlorine is more than triple as effective at killing algae at 7.2 as it is at 8.0.
- Set the shock target off cyanuric acid, not a fixed number: raise free chlorine to roughly 40% of the CYA reading and hold it there.
- Brush the pool and run the filter around the clock while you shock - dead algae is what clouds the water, and the filter clears it.
- Expect 1-2 days for a light bloom and 3-5 days for a deep green; retest every few hours and re-dose until chlorine holds overnight.
- When CYA is over 100 or the water is a black swamp, a partial drain of 30-50% is faster and cheaper than chlorine alone.
- Prevent it by holding free chlorine at 5-7.5% of the CYA level on every visit - the pools that turn green are the ones running low chlorine against high CYA.
How do I chemically treat a green pool?
Treat a green pool by killing the algae with a sustained high dose of chlorine while the water is chemically primed to let that chlorine work. Green water is an algae bloom, and algae only dies when free chlorine is held well above its normal range long enough to oxidize all of it. The order of operations is what separates a fast clear from a stalled one: balance first, shock hard, then keep the chlorine up and the water moving until it runs clear.
This is the sequence that clears the large majority of green pools without draining a drop:
- Test first. Get free chlorine, pH, and especially cyanuric acid (CYA), because CYA sets how much chlorine a shock actually needs.
- Lower pH to about 7.2. Chlorine kills algae far faster in slightly acidic water, and a lower pH keeps the shock from clouding the pool.
- Shock to the target level. Raise free chlorine to roughly 40% of the CYA reading - for a pool at CYA 30, that is about 12 ppm - using liquid chlorine or cal-hypo.
- Brush the walls and floor. Knocking the algae loose exposes it to the chlorine instead of letting it hide in a shaded corner.
- Run the filter 24/7 and clean it as it loads up. Dead algae is what clouds the water, and the filter is what pulls it out.
- Retest and re-dose every few hours. Free chlorine burns off fast against a heavy bloom, so keep dosing it back to target until the green is gone.
Match the shock dose to how green the pool is
How hard you shock depends on how far the bloom has gone, and you can read that off the water before you test anything. A pool that is cloudy and light green is a young bloom you can often clear in a day or two. A deep, opaque green you cannot see the second step through is an established bloom that takes several days and multiple shock doses. Water gone dark green to near-black, with surface scum, is a swamp - clearable, but it is a week of work and a real chemical bill, and it is the point where a partial drain starts to compete with chlorine on cost.
The table below is the field read. Match the color to the approach, and set the customer's expectations on clear time up front, because a green pool almost never clears the same day and promising that is how you end up with an angry account.
| Water color | What it means | Chlorine approach | Typical clear time |
|---|---|---|---|
| Cloudy, light green | Young bloom, chlorine just dropped | One shock to target, hold it | 1-2 days |
| Deep, opaque green | Established bloom | Shock to target, re-dose every few hours | 3-5 days |
| Dark green to black, scum | Swamp - heavy organic load | Repeated shocking; consider partial drain | 5-7+ days or drain |
Balance pH and alkalinity before you shock, not after
Drop the pH to about 7.2 before you add the first dose of chlorine, because chlorine is far more effective as a sanitizer in slightly acidic water. At a pH of 8.0, only about 20% of your free chlorine is the active, algae-killing form (hypochlorous acid); at 7.2 that climbs to roughly 65%. Same chlorine number on the test kit, more than triple the killing power. Shocking a high-pH pool wastes chlorine and leaves the bloom half-fought.
Do not chase perfect alkalinity or calcium in the middle of a bloom - those are slow readings that do not affect the kill. Get total alkalinity roughly into the 80-120 ppm band so pH holds steady, set pH at the low end, and move on to the shock. Once the water is clear and holding chlorine overnight, then you rebalance for stability using the saturation index (LSI) so the pool you just cleared does not turn around and etch or scale. Balance for the kill first, balance for the long term after.
Why pools turn green in the first place
A pool turns green when free chlorine drops far enough, for long enough, that algae blooms faster than the sanitizer can kill it - almost always a chlorine problem, not a mysterious one. The usual triggers are a chlorinator that ran empty, a pump that was off, a stretch of 100-plus-degree days that burned chlorine faster than usual, or a cyanuric acid level so high that the chlorine present was effectively locked up. Heat and sun are the accelerant: a Sunbelt pool can go from clear to green in 48 hours in July.
Take a solo operator who picks up a foreclosure account in Mesa in July. The pool has been off two weeks in 108-degree heat, the water is opaque green, and CYA tests at 110 ppm from a season of neglected tablets. That last number is the trap: at CYA 110, holding shock level means dragging free chlorine to 40-plus ppm, and sizing that much chlorine off the pool's gallons takes gallons of liquid chlorine and days of dosing. This is the case where a half-drain and refill to knock CYA down to 30-40 first makes the whole job faster and cheaper than chlorine alone. Reading the CYA is what tells you which fight you are actually in.
When chlorine alone is not enough: mustard algae, phosphates, and draining
Most green blooms are common green algae and die to chlorine alone, but a few cases need more. Mustard (yellow) algae clings to walls and shaded spots and resists normal shock levels, so it needs a higher, longer hold plus hard brushing, and often a dedicated mustard-algae treatment. Black algae roots into plaster under a waxy cap; you have to knock the heads off with a stiff brush so chlorine can reach the roots, and it is the most stubborn of the three to clear.
Phosphates get blamed for green pools, but they are algae food, not the cause - a pool with high phosphates and adequate chlorine stays clear. A phosphate remover is worth it only on a pool that keeps re-blooming despite good chlorine, where levels run above about 1,000 ppb. And when CYA is over 100 or the water is a true swamp you cannot see into, a partial drain and refill is the right call: replacing 30-50% of the water resets CYA and cuts the organic load so the chlorine you add actually works, instead of pouring chemicals into a pool that will not respond.
Keep a route from turning green: track free chlorine against CYA
The way you keep a whole route out of the green is to hold free chlorine at the right ratio to cyanuric acid on every pool, every visit - a maintenance minimum of roughly 5-7.5% of the CYA level. A pool at CYA 50 needs free chlorine kept near 3-4 ppm, not the 1 ppm that looks fine on a strip; let it ride at 1 ppm against high CYA and it is a bloom waiting for the next hot week. The pools that turn green on a route are almost always the ones running low chlorine against creeping CYA.
This is a tracking problem before it is a chemistry problem. When you log chemical readings per pool at every visit, the accounts drifting toward trouble show up as a trend - CYA climbing, free chlorine sliding - weeks before the water turns. An operator watching the readings they log at every stop catches the pool that needs a CYA reset or a switch off stabilized tablets in spring, instead of clearing a full-blown green pool in July. Prevention costs a reading and a note; a green pool costs a Saturday and a chemical bill.
FAQ
Frequently asked questions
Is a green pool safe to swim in?
No, a green pool is not safe to swim in, and the same low chlorine that let the algae bloom means bacteria are not being controlled either. Algae itself is usually not dangerous, but it makes the water opaque enough that a swimmer in trouble can disappear from view, and a green pool almost always tests at or near zero free chlorine, so pathogens like E. coli and the parasites behind recreational water illness can survive. There is also a real slip hazard, because algae on the steps and floor is genuinely slick. Keep swimmers out until the water is clear, free chlorine has burned back down into the normal 1-3 ppm range, and combined chlorine reads under 0.5 ppm. For a service company, that is worth telling the customer plainly - a family that swims in a green pool because it looks fine now is a liability conversation you do not want.
Why is my pool still green a day after I shocked it?
The most common reason a pool is still green a day after shocking is that free chlorine did not stay at shock level - it got eaten by the algae within hours and dropped back down, so the kill never finished. A heavy bloom consumes chlorine fast, which is why the job is not one dose but holding the target level with repeated doses over 2-5 days. Retest a few hours after your first shock: if free chlorine has fallen well below target, the pool just told you it needs more. Other culprits are a dirty filter that cannot clear the dead algae, a pH that crept back up and cut the chlorine's effectiveness, or a cyanuric acid level so high your shock was never actually at shock level for that CYA. Confirm the target against the CYA reading and hold it - persistence, not a single bigger dose, is what clears a stubborn bloom.
Can I use an algaecide instead of chlorine to clear a green pool?
No - algaecide is a preventative and a supplement, not a primary treatment for a pool that is already green. Chlorine is what actually oxidizes and kills an established bloom; most algaecides are designed to stop algae from taking hold in water that is already sanitized, and dosing a green pool with algaecide alone typically wastes money and can foam the water. There is a narrow exception: a specialized polymer or copper-based algaecide can help as a follow-up on stubborn mustard or black algae after you have shocked. Some operators also add a maintenance dose of algaecide as cheap insurance heading into a hot stretch. But the sequence is always chlorine first. If a pool is green, shock it to the target level off its CYA reading and hold it - the algaecide, if you use one at all, comes after the water is clear to help keep it that way.
How much does it cost to clear a green pool?
Clearing a typical residential green pool costs roughly $30-$80 in chemicals for a light-to-moderate bloom, mostly liquid chlorine or cal-hypo, plus your labor and return trips. A light green pool might take a couple of gallons of liquid chlorine and clear in a day or two. A deep green pool can run $80-$150 in chlorine because you are holding shock level for several days and re-dosing repeatedly. A true swamp, or a pool with cyanuric acid over 100 that fights every dose, is where the cost climbs fast - and where a partial drain and refill (a water bill of maybe $30-$60 in most areas) often comes out cheaper than the chlorine it would take to shock it at that CYA. Price a green-pool recovery as a one-time job separate from the regular service rate, because the chemical and the time are both well beyond a normal visit.
Will a green pool stain the plaster or damage the equipment?
A short green spell rarely stains, but a pool left green for weeks can stain plaster and damage equipment, so time matters. Algae itself brushes off most surfaces once you kill it, but a heavy bloom that sits for a month can etch into plaster, and black algae actually roots into the surface and leaves spots even after treatment. The bigger risk is what low-chlorine, unbalanced water does while it is green: it can corrode metal in the heater and pump, and dead algae clogs the filter and can burn out a pump running against blocked flow. There is also a chance of metal staining when you shock hard, because high chlorine can pull iron or copper out of solution and onto the walls. The move is to clear it quickly, keep the water balanced during the shock, and check the filter and heater afterward on any pool that was green for a long time.
How do I know when the algae is gone and it is safe to stop shocking?
The pool is truly clear when it passes an overnight chlorine loss test, not just when it looks blue again. Water can go clear while a little algae is still alive, and it will bloom right back if you stop shocking too soon. To test it, at dusk bring free chlorine up to your shock target, record the number, and add no more chlorine overnight. Retest at first light before the sun hits the water: if you lost 1 ppm or less, the algae is dead and you can let chlorine drift back to normal. If you lost more than that, something is still consuming chlorine, so you keep shocking. Combine that with the visible checks - the water is clear enough to see a coin on the bottom, and combined chlorine reads under 0.5 ppm - and you can hand the pool back to the customer with confidence instead of guessing.
