Vacuuming green algae from the bottom of a pool: the step-by-step method, in waste mode

The shock treatment has worked: the water has cleared, but the bottom of the pool is covered with a greenish-gray film, sometimes a real powdery layer that rises with the slightest movement. These are dead algae mixed with flocculant and fine particles. To remove them properly, you need to vacuum them slowly with a manual vacuum, with the filter valve set to the waste (or drain) position, so they are sent directly to the drain instead of passing through the filter. It is a simple operation, provided you follow the steps in the right order. Here is the complete method, as we recommend it in the store.

Why you shouldn't vacuum algae on the filter setting

During routine maintenance, you vacuum the bottom with the multiport valve set to the filter position: the water sucked up passes through the filter and returns clean to the pool. When algae have settled, this method causes two problems. First, dead algae and flocculant form very fine particles that quickly clog the sand or cartridge; the pressure rises, the flow rate drops, and you spend your time backwashing the filter. Second, some of these particles pass through the filter media and return to the pool through the return jets. You move the problem instead of eliminating it.

The waste setting (also called drain or “waste,” depending on the valve) bypasses the filter: everything the vacuum cleaner sucks up goes directly to the drain. You lose water, but you genuinely remove the algae. That is the key principle to remember.

Step 1: treat before vacuuming

Vacuuming live algae is pointless: they quickly grow back on the walls. Vacuuming is the last step of a treatment, not the first. Start by vigorously brushing the walls, steps, corners, and bottom. Brushing removes the algae film and makes it accessible to the chlorine. Then adjust the pH: chlorine is significantly more effective in water around 7.0 to 7.4 than in water with a high pH. Then carry out the shock treatment at the dosage indicated by the product manufacturer for your pool volume, and keep the filtration running continuously for as long as necessary.

If the water remains cloudy or greenish after the algae have been killed, a flocculant suited to your type of filter groups the fine particles into heavier flakes that sink to the bottom. This is precisely what makes vacuuming in waste mode so effective: instead of a cloud in suspension, you have a layer to collect.

Step 2: let it settle

After the shock treatment (and any flocculant), turn off the pump and leave the water completely still, ideally for an entire night. The dead algae will settle and form an even layer. Avoid swimming, using the robot, or restarting filtration before vacuuming: every movement puts the deposit back into suspension. The key to successful vacuuming largely lies in the previous day: the calmer the water, the more compact and easier to remove the deposit.

Step 3: prepare the equipment and water level

Mount the vacuum head on the telescopic pole and connect it to the floating hose. Since you will be discharging water, first raise the level a few centimeters above normal, without excessively exceeding the top of the skimmer opening. Prepare a garden hose to top up the water during the operation. Also check where the water from your discharge goes: when discharging to waste, water containing chlorine and flocculant must flow into the designated drainage system, not the garden or a ditch. Local discharge regulations may vary: check if in doubt.

Step 4: prime the hose without air bubbles

A hose full of air causes the pump to lose its prime and run dry. To prevent this, lower the vacuum head to the bottom, then gradually immerse the hose in the water, meter by meter, until it is completely full. Another method: hold the free end in front of a return jet, with the pump running in filter mode, until no more bubbles come out of the vacuum head. Then block the end with your hand, bring it to the skimmer (or vacuum port), and connect it underwater.

Then stop the pump, turn the multiport valve to the waste position, and only then restart it. Never operate the valve while the pump is running: you risk damaging the valve spider gasket.

Step 5: vacuum slowly in parallel strips

This is where most mistakes occur. The vacuum head must move very slowly, roughly at the pace of a person walking in slow motion. A quick movement creates a wave in front of the vacuum head, lifting the sediment into a cloud; you will then have to wait many hours for it to settle again. Work in slightly overlapping parallel strips, starting from the area farthest from the skimmer, and return without lifting the vacuum head from the bottom.

• Keep an eye on the water level: in waste mode, it drops quickly. Leave the garden hose running continuously.
• Never let the water level drop below the skimmer opening: the pump would draw in air.
• If a cloud forms anyway, stop, let it settle for a few minutes, then resume more slowly.
• For heavy deposits, do several short sessions rather than one very long one.

Step 6: after vacuuming

Stop the pump, return the valve to filter mode, then backwash the sand filter followed by rinsing, or carefully clean the cartridge. Restore the water level to its usual height. The fresh water added dilutes the chemicals: check the pH and disinfectant level the same day, then again the next day. Run the filtration longer than usual for a few days, until the water regains perfect clarity.

And what about the robot?

An electric robot is perfect for regular maintenance, but it is not the ideal tool for a heavy deposit of dead algae and flocculant. Its internal filter (bag or basket) becomes saturated very quickly with particles this fine, and some of them pass back through. It is better to remove most of the deposit manually with a vacuum head in waste mode, then let the robot resume its usual work on a clean pool. Autonomous pool vacuum cleaners that discharge the extracted water outside can also be useful when the pool does not have a usable waste setting.

How much water is lost in waste mode?

The flow rate of a household pool pump is measured in cubic metres per hour. In waste mode, several hundred litres can therefore be discharged in just a few minutes. During a thorough vacuuming session, the loss generally remains around a few centimetres of water level, which you can make up with a garden hose as you go. To limit waste, vacuum only the areas that are actually heavily soiled, starting with the thickest deposits, and stop the pump between areas while you reposition the vacuum head. On a very dirty pool floor, two short sessions a few hours apart are preferable to one long vacuuming session that would excessively empty the pool.

Fresh water has another advantage: it dilutes some of what accumulates over the season, particularly the stabilizer introduced by stabilized chlorine products such as dichlor or trichlor. Water with too much stabilizer makes chlorine less active, which is precisely what encourages algae to return. If you use these products regularly, checking the cyanuric acid level at the end of the process is a good idea.

Preventing the green carpet from returning

A pool that turns green is always sending a message: disinfection has not kept up with the water's load. The usual causes are filtration running for too short a time in hot weather (the practical rule is that the daily duration should increase with the water temperature), pH drifting too high, chlorine levels remaining too low for several days, or nutrients entering through rain, leaves and pollen. After a successful vacuuming, get into the habit of brushing the walls once a week, checking pH and chlorine twice a week in summer, and regularly emptying the skimmer and pump baskets. These simple steps take far less time than another vacuuming session to waste.

Mistakes that cost you a day

• Vacuum before killing the algae: they grow back within a few days.
• Vacuum a thick deposit while filtering: clogged filter, water becoming cloudy again.
• Go too fast: all the deposit gets stirred back into suspension.
• Forget to top up the water: the skimmer draws in air and the pump loses its prime.
• Do not correct the cause: filtration time too short, pH out of balance, insufficient disinfectant or abundant phosphates; without correcting it, the algae will return.

For the actual treatment of green water, we have covered it in detail in our guide to treating green pool water. And if you are unsure how to connect the vacuum head, the step-by-step process is explained in how to connect the pool vacuum head to the skimmer. For an older layer of sludge at the bottom of a pool, the process is similar: see how to clean sludge from a pool.

Read also

Before vacuuming, address the cause: How to treat green water in a swimming pool?.

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Frequently asked questions

— Should algae be vacuumed in waste or filtration mode?

For a thick deposit of dead algae and flocculant, set the valve to waste: the water goes directly to the drain without clogging the filter. For a very light film, filtration followed by backwashing the filter may be sufficient.

— How long should I wait after shock treatment before vacuuming?

Filter for the time indicated by the product manufacturer, then stop the pump to let it settle, ideally overnight. Vacuum the next day, when the water is still.

— Why does the algae deposit rise when I vacuum?

The vacuum head moves too quickly and creates a wave that lifts the particles. Slow down considerably, work in parallel strips, and pause if a cloud forms.

— Can a robot be used to vacuum up dead algae?

For a large deposit, it is not ideal: its filter quickly becomes saturated and lets fine particles through. First remove the deposit with a manual vacuum head, then resume maintenance with the robot.

In summary

• Vacuum the algae only after brushing, correcting the pH, and carrying out shock treatment.
• Let it settle overnight with the pump off; this makes the deposit compact and easy to remove.
• Thick deposit: set the valve to waste, prime the hose without air, and top up the water during the operation.
• Move the vacuum head very slowly in parallel strips, without leaving the bottom.
• Next: set the valve to filtration, backwash the filter, and check the pH and chlorine.

Last updated: October 3, 2026.