Green water: what belongs to cleaning the pool, and what belongs to treating the water

The green you can see is microscopic algae in suspension. It is present in every pool in the world; what has changed is that it has been allowed to multiply. Four conditions make that possible, and they almost always arrive together.

• A sanitiser that has dropped too low. Sustained heat, an unusual number of swimmers, a thunderstorm, or simply a few days away are enough to burn through the reserve.
• A pH that has climbed too high. This is the factor most often overlooked, and the next section is devoted to it: above a certain value, the sanitiser sitting in the water loses most of its effect. The test reads a correct level, and yet nothing works.
• Too short a filtration run. Without enough circulation the water stagnates, matter is never captured and the product never distributes itself.
• Nutrients blown in from outside. Dust, pollen, leaves, sand, irrigation splash, soil from planters, sun cream. That organic matter is what algae feed on.

The point that matters: treating one of those causes on its own never holds. It is what explains the classic sequence — the water clears for three days, then goes green again. The symptom was treated, not the condition that allowed it.

What follows describes what is happening in the pool, and who to call.

This is the technical point that explains most failures, and owners rarely have it explained to them.

The instinctive reasoning is: "there isn't enough product in there, I'll add some". Yet in a great many cases the product is there and it is not working, because the pH of the water is too high.

A chlorine-based sanitiser exists in water in several forms depending on acidity, and only some of those forms are actually active. As pH rises, the active share collapses: the reading stays good, the real effect does not.

What that means in practice, in this order:

1. measure and correct the pH first;
2. adjust the sanitiser next;
3. and only then judge the result.

Doing it the other way round means pouring product into water where it will not work — which is expensive, saturates the water and, in the end, forces you to renew it.

Two local factors push the pH up in Marrakech: hard fill water, rich in minerals, and heavy evaporation that concentrates those minerals right through the summer. That is not owner neglect, it is the climate.

Those measurements and corrections belong to a pool technician, or to an owner with the right kit. What matters is understanding the mechanism, because without it the rest of this page makes no sense.

When water goes green again despite correct treatment, this is where the cause sits in the large majority of cases.

The situations that keep coming up:

• too short a running time for the temperature of the water. In full Marrakech heat, a few hours a day are not enough to pass the volume of the pool through the filter;
• a fouled or saturated filter, whose pressure reading says it is no longer filtering much of anything;
• full skimmer and pump baskets, which cut the flow rate to almost nothing;
• a night-only schedule, when demand peaks precisely when the water is warm and the pool is in use;
• a pool with poorly distributed circulation, with dead zones — corners, steps, under the ladder — where the water does not move and where the green appears first.

One simple, free clue: look at where the green starts. If it shows up first in the corners, on the steps and behind the ladder, it is a circulation question. If it appears evenly through the whole body of water, it is a disinfection question.

Here too, the settings, the upkeep of the filter and restarting the installation belong to your pool technician, who knows the pump, the filter and the valves of your installation.

This is the local variable that generic guides ignore, and it explains why a Marrakech pool goes green faster than one elsewhere under equal conditions.

The chergui — the hot easterly wind that blows mainly from July to September — brings three things at once:

• a mineral load — a very fine sand that gets through the baskets, fouls the filter and settles on the floor;
• an organic load — plant dust, pollen, debris, all of it a direct supply of nutrients;
• accelerated consumption of the sanitiser, spent oxidising all of the above.

To which a villa garden adds its everyday contributions: irrigation throwing up soil, planters at the edge of the beach area, leaves from a nearby tree, and building dust when there is a site next door.

Two practical consequences:

• the risk window can be identified in advance, which makes it possible to plan rather than react;
• a cover during windy episodes sharply reduces what goes in, and is probably the best value measure on this whole page.

A pool left unusable through that period is also a considerable mosquito breeding ground as soon as the filtration stops — a subject covered in our article on mosquitoes around a pool.

This is the only genuinely expensive decision, and it is taken on criteria, not on appearance.

Rescuing the water is worth considering when the water is green but translucent — you can make out the floor, or at least the outline of the steps — when the filtration installation is in a fit state to run, and when the episode is recent. It is water treatment work, it takes several days, and it belongs to a pool technician or to an owner with the right kit.

Draining is the better answer when:

• the water is opaque, dark green or brown, and the floor is no longer visible at all;
• the floor is covered in a thick deposit of sand, decomposed leaves or silt;
• the water is chemically saturated after months of successive corrections;
• the pool has stood idle for several months, often in an unoccupied house;
• a repair to the finish, to the built-in fittings or to the waterproofing is needed in any case.

A word of caution: draining a pool is not a neutral act. A full pool is balanced by the water it holds; drained at the wrong moment — waterlogged ground, high water table, a séguia (traditional irrigation channel) nearby, after heavy rain — it can lift or take the thrust of the surrounding earth.

That is a structural claim, not an incident. The timing and the method of draining are therefore decided with the pool technician, who knows the structure and the ground.

On a drained pool, there is nothing chemical left about the work: it is a mechanical cleaning job, exactly like a floor after building work. Here is the order to expect from whoever takes it on, pool technician or a firm that makes it its trade:

1. Clearing the residual water with a water vacuum. A few centimetres at the low point are not an obstacle.
2. Removing deposits before any brushing: sand, decomposed leaves, silt, debris. Brushing a dry crust wears out the brush and scratches the surface without lifting anything.
3. Mechanical brushing of the floor, the walls and the steps, with the pad and brush matched to the finish — tile, mosaic, zellige (traditional glazed terracotta), paint, liner.
4. Separate treatment of the waterline, the hardest band in the pool, where limescale and greasy matter have combined.
5. Restoring the coping stones, the only part anyone touches barefoot, with a product matched to the material.
6. Full rinse and removal of the rinsing water.

One important point about the green: on a drained pool, the green on the walls is no longer living algae. It is a dried residue keyed into the microporosity of the finish and into the grout lines, and it does not come off with an algaecide but mechanically.

Before approving a quotation, check that these six steps are on it and that the finish has been identified: the finish decides the pad, the brush and the product.

Better written down here in black and white than discovered on the phone.

The pool technician:

• water treatment: disinfection, pH correction, algaecides, flocculant, mineral salts;
• balancing and monitoring the water, at whatever interval;
• filtration: pump, filter, valves, scheduling, repairs;
• the timing of the drain, and the mechanical cleaning of the drained pool;
• repairs to the finish, the built-in fittings, the waterproofing or the lighting;
• a season maintenance contract, if you want one.

The owner or their staff: daily monitoring, the baskets, the cover during windy episodes, and usually the refilling.

This is not commercial modesty. A pool whose water has been rescued three times by someone who does not have the trade ends up drained anyway, with a damaged finish thrown in. One good contact per task costs less than a jack of all trades.

Green water in June is nearly always the consequence of a May that never happened.

• February to March. Check the installation before the temperatures climb: filter, baskets, valves, lighting. This is the moment to decide on a possible drain, while the ground is still favourable.
• April. If the pool is to be drained, it is now. Mechanical cleaning is done on an empty pool, before filling — the pre-season preparation is detailed in our article on getting a pool ready before summer.
• May to September. Close monitoring, longer filtration runs as the temperature rises, a cover during windy episodes.
• October. Catching up after the chergui season, once the deposit stops rebuilding itself. It is the best time of year for a mechanical intervention.

This calendar does not replace water monitoring — it simply makes it less acrobatic. Before calling your pool technician, note down the pool's surface area, its finish, the condition you have seen and the planned draining date: those are the first questions they will ask.