How to Fix a Green Pool: The Professional Green-to-Clean Procedure

Somebody calls on a Monday and says their pool has gone green. What they want to know is whether it can be saved or whether it has to be drained. Almost always it can be saved, and almost always draining is the wrong answer: an empty vinyl liner shrinks and will not go back, an empty fibreglass shell can float out of the ground where the water table is high, and a plaster pool emptied in hot sun can craze. The water is not the problem. The sanitiser that stopped holding is the problem, and green is what that looks like about four days later.

For anyone running a route, a green recovery is also one of the few jobs that reliably earns more per hour than a weekly clean, and one of the easiest to lose money on. The chemistry is not difficult. The difficulty is that the job takes several visits, most of them short, and an operator who quotes it as one afternoon has already priced the return trips out of their own pocket. This is the procedure a professional follows, and then how to make it pay.

Green water is a chlorine failure, not an algae infestation

Algae spores are in every pool, all the time. They blow in, they come off swimsuits, they arrive on a leaf. In a pool carrying a normal free chlorine residual they never get anywhere, because the chlorine oxidises them faster than they can divide. Green water is the visible evidence that the residual went to zero and stayed there long enough for the spores to win. Usually the cause is boring: the pump was off for a week, the chlorinator ran empty, a heavy rain diluted the pool while filling it with organics, or a filter clogged and flow dropped far enough that the water stopped turning over.

That matters for how you approach the job. If you treat the green as the problem, you dump algaecide in, the water clears slightly, and it is green again in five days. If you treat the chlorine as the problem, you find out why the residual failed, fix that, and then remove the algae that grew while it was failing. The second approach is the one that does not bring you back for free.

Chlorine demand

The amount of chlorine a body of water will consume before any free residual appears. A green pool is full of organic material that has to be oxidised first, so the first several pounds of shock produce no measurable free chlorine at all. This is normal and is why partial dosing fails: you spend the chemical, satisfy none of the demand, and the pool looks unchanged.

Chlorine demand is the single most common reason an owner's own attempt fails before they call you. They added one bag of shock, saw nothing happen, concluded the pool was beyond saving, and stopped. The pool was not beyond saving. One bag was simply less than the demand, and everything they added was consumed satisfying it.

What the owner already tried, and what it left behind

By the time a route operator gets the call, the pool has usually had a week of amateur chemistry applied to it, and what went in changes what you are standing in front of. Ask before you test. Three answers matter: how much shock was added and when, whether tablets have been running continuously in a floater or feeder, and whether anything sold as an algaecide or a clarifier was used.

Tablets are the answer that quietly ruins the plan. Stabilised tablets carry cyanuric acid with them, and an owner who responds to a failing pool by loading the floater has raised the stabiliser level a little more every day they did it. At high cyanuric acid the chlorine you add still shows on the test but does far less work, which is why an owner can truthfully report adding plenty of chlorine and still be looking at green water. That one reading decides whether the pool can be shocked into submission or whether some of the water has to be replaced first, and it is the difference between a job you quote in visits and a job you quote in truckloads.

Algaecides are the second trap. The cheap ones are copper based, and copper sitting in a pool that is about to be shocked hard will stain plaster in a way that does not brush out. The owner then has clear water with grey-green marks on the steps and a grievance, and the marks arrived on your visit. Quaternary algaecides do not stain but foam badly once the pump runs. Neither removes algae that is already established; both suppress growth in water that is already sanitised. If either has gone in, record it before you start, because it is precisely the kind of pre-existing condition that a written scope should already cover.

Test before you dose: four readings decide the whole job

Test strips are not adequate here. Green water throws colour comparisons badly, and two of the four readings you need are outside a strip's useful range. Take a sample at elbow depth away from the returns and run it on a drop-based kit. If the pool is dark enough that you cannot judge the colour blocks, dilute the sample with a known volume of tap water and multiply back.

The green-to-clean procedure, in order

The order matters more than any individual step. Every stage exists to make the next one work, and skipping one usually means repeating the one before it.

1. 1

   Get the debris out first

   Net out leaves and heavy debris with a deep-bag leaf rake on a telescopic pole, working from the deep end toward the shallow. Empty the skimmer and pump baskets. Every pound of organic matter you remove by hand is chlorine demand you do not have to pay for.

2. 2

   Confirm the equipment actually runs

   Start the pump and watch it prime. Check the pressure gauge against the clean starting pressure, backwash or clean the cartridge if it is already high, and look for air in the pump pot. If the pool cannot circulate, nothing after this point works, and the equipment failure is very often the reason the pool went green in the first place.

3. 3

   Correct pH and alkalinity

   Bring pH down into the 7.2 to 7.4 band and get alkalinity into range before any chlorine goes in. Add acid slowly with the pump running and give it a full turnover before retesting. This is the step people skip, and it is the one that decides how much of your shock does any work.

4. 4

   Brush every surface

   Use a wide flat-headed pool brush on a pole and cover walls, floor, steps, the waterline tile and behind the ladder. Algae clinging to plaster is physically shielded from the water you are about to treat. Brushing puts it into suspension where the chlorine can reach it, and it is the cheapest thing you do all day.

5. 5

   Shock hard, once, and at dusk

   Dose to a genuine shock level for the pool's volume, following the label on the product you carry rather than a rule of thumb, and add it in the evening so sunlight is not burning it off while it works. One decisive dose beats three timid ones, because the demand has to be satisfied before any residual appears. Keep the pump running continuously from this point.

6. 6

   Hold the residual overnight, then retest

   The next morning the pool will usually be cloudy grey or milky blue rather than green. That colour change is the win: it means the algae is dead and what you are looking at is the debris. If it is still green, the demand was higher than the dose and it needs shocking again.

7. 7

   Filter, clean the filter, filter again

   This stage is where the clock actually goes. Dead algae is fine enough to blind a filter quickly, so expect pressure to climb fast and to backwash or rinse a cartridge several times. A clarifier or a flocculant can shorten it considerably, at the cost of an extra vacuum.

8. 8

   Vacuum the settled load out

   Whatever has dropped to the floor comes out with a manual vacuum, slowly, so you are not stirring it back into suspension. Whether that goes through the filter or straight to waste is the decision in the next section.

9. 9

   Rebalance and hand it back

   Once the water is clear, bring pH, alkalinity and calcium hardness back into normal range, set the stabiliser right if you had to dilute, and establish the ongoing chlorination the pool will run on. Then fix the original cause, whether that is a timer, a chlorinator setting or a filter that needed servicing.

Vacuum to waste, or through the filter?

On a multiport valve you can send the vacuum discharge out of the waste line instead of through the filter, so the dead algae never touches the media. It costs you water, because everything you pull out has to be replaced, and it drops the level fast enough that you can lose prime if you are not watching. Cartridge filters have no waste port at all, which settles the question for a lot of residential pools.

The practical rule is that a light haze goes through the filter and a floor covered in settled grey sludge goes to waste. If you are refilling substantially, retest everything afterwards, because you have just diluted the pool with whatever comes out of the tap.

Not every green pool is the same algae

Most green pools are ordinary green algae suspended in the water, and the procedure above handles them. Two other kinds behave differently enough that treating them the same way produces a pool that goes green again within a fortnight, and an owner who now thinks you did not fix it.

Quoting a green-to-clean so it actually pays

The mistake is quoting a price for the afternoon you are standing there. A green recovery is a visit to assess and shock, a visit to check and re-shock, one or two visits to clean the filter and vacuum, and a final visit to rebalance. That is three to five trips to the same address inside a week, most of them twenty minutes long, and the driving between them is the real cost. Build the number the same way you would build any recurring service rate: time on site, drive time, chemical at what it costs you today, and the margin you need.

Chemical is the part that most often goes wrong, because the quantity depends on the pool's volume and on how far gone it is, and you cannot know the second one precisely until you have shocked it once. The honest way to handle that is to quote labour firmly and chemical as an allowance, stated up front, with anything beyond the allowance billed at cost plus your markup. Customers accept this readily when it is explained before the work rather than discovered on the invoice.

Put the stopping conditions in writing too. If the plaster turns out to be stained rather than dirty, if the filter is beyond cleaning, or if cyanuric acid forces a partial drain, those are changes in scope and not part of what you quoted. The same clauses that protect a weekly service agreement protect a one-off recovery, and a recovery is exactly the job where an unclear scope turns into an argument.

Fitting a recovery into a route week

The scheduling problem is that a recovery does not fit the shape of a route. A route is one long efficient loop; a recovery is several short returns to one address. Squeezed into the gaps of a normal week it quietly destroys the day's density, because each return trip is a detour off the loop rather than a stop on it.

Batching only works if the recovery is visible when the week is planned rather than discovered on the morning. That makes it a scheduling problem more than a chemistry one, and the same problem as sequencing any set of stops by geography instead of by the order the calls came in. Booking the return visits at the time you quote, on days you are already in that part of town, costs nothing and is the whole of the difference.

There is a commercial reason to get this right beyond the one job. A recovery customer is a customer who has just proved they will not maintain the pool themselves, and they have watched you fix something they could not. It is the easiest conversion to weekly service you will ever get, and the easiest moment to set a rate that reflects what the work is worth rather than inheriting whatever the last person charged. Owners who are otherwise reluctant to raise a price find it far easier to set the right one at the start.

Stopping the same pool going green again

A pool that has gone green once will go green again unless the reason it happened is dealt with. On the final visit, work through the cause rather than just the symptom, and write down what you found so the next technician on that account knows the history.

Start at the pump. Ask how many hours it runs, then check whether it is actually running for them, because a timer that has drifted or a breaker that trips on hot afternoons produces exactly the pattern that lets a residual collapse without anyone noticing. The whole volume needs to pass through the filter at least once a day, and more than that in summer heat. An owner who cut the pump back to four hours to save electricity has usually built the conditions themselves.

Then read the pressure gauge on the filter head and compare it against the clean-media baseline for that filter. Eight to ten psi above clean means flow has dropped far enough to matter, and after a green recovery the filter will reach that within days because the media is loaded with dead algae. If nobody knows the baseline, set one on your final visit and write it on the equipment pad and into the account record. Without it the gauge is a number with nothing to compare it to, which is why so many pools run for months on a filter everyone assumes is fine.

Last, look at how chlorine is actually being delivered. An empty inline feeder, a floater that has been sitting on the steps for a month, or a salt cell that has scaled over or been left at a winter output setting are all common enough to check by default. A cell that has lost output will keep reporting that it is generating while producing very little, so trust the free chlorine reading rather than the panel. Fixing the delivery is usually cheap, and it is the difference between a recovery that holds and one you repeat on the same account next season, which is worth carrying into the rate you set for the ongoing service.

That last item is worth taking seriously. Mustard algae in particular travels on equipment, and an operator who works a yellow-algae pool in the morning and a healthy account in the afternoon with the same unrinsed brush has just created next month's problem. Carrying a spare brush head and a dedicated net for recovery work is cheap insurance, and it is one of the reasons how a truck is laid out matters more than it looks like it should.

In practice that means a recovery kit that never mixes with the weekly kit: its own brush head, its own net, its own bucket, rinsed and stored apart. It is a small amount of discipline and it is much easier to hold to when the truck is laid out by how often each tool is touched rather than by whatever fitted when it was loaded.

None of this is complicated work. It is patient work, spread across several short visits, where the difference between a profitable job and an unpaid one is whether the visits were planned and recorded or simply absorbed. Keeping every reading, every return trip and every chemical cost attached to the job is what lets you quote the next recovery from evidence instead of memory, and it is the same discipline that keeps an ordinary route week from quietly eroding.