Deposits at the bottom of the pool: identify each deposit and remove it using the right method

In the morning, the water looks clear, but the bottom tells a different story: a fine gray dust that billows up at the slightest movement, a small pile of sand in a corner, a yellowish trail on the shaded side, and a few waterlogged leaves near the drain. You vacuum it up, and two days later it has all come back. To permanently remove deposits from the bottom of a swimming pool, you first need to know what they are made of: mineral or organic deposits, algae, and chemical precipitation cannot be treated with the same tool or the same filtration setting. Here's how to make the right diagnosis, then clean the bottom without sending the dirt back into the pool.

What settles on the bottom, and why everything ends up there

A swimming pool works like a large settling tank. Water is drawn in at the surface by the skimmers, sometimes from the bottom through the drain, filtered, and then returned through the return jets. Between the jets and the suction points, there are areas where the water moves very little: corners, the bottoms of steps, the base of the walls, and the diving well. Anything heavier than water eventually settles there, and once it is on the bottom, surface filtration can no longer catch it.

At night, when the pump is stopped, sedimentation is even more apparent: in the morning, you discover dust carried by the previous day's wind, pollen, and grains of sand brought in on people's feet. After a storm, it's soil and plant debris. And when the water itself becomes unstable, dissolved minerals or algae come to coat the bottom.

So this is not a defect in your pool: it is a normal physical phenomenon. The key is to distinguish deposits that simply require cleaning from those that indicate a water or filtration problem.

Identify the deposit before taking out the vacuum

Take a few minutes in the morning, before filtration puts the water back in motion. Observe the color, location and reaction of the deposit when you touch it with the brush.

The most revealing test is still the brush test: a deposit that lifts in a cloud and settles again is a cleaning matter; a deposit that does not move is a water chemistry matter. In the first case, the right tool is enough. In the second, vacuuming will be useless until the cause has been addressed.

The equipment: which tool for which deposit

There is no universal tool. Each device has its strengths, and most owners end up combining two or three solutions.

• The deep bag leaf net, mounted on a telescopic pole: essential for leaves, insects and coarse debris. Always use it first, otherwise leaves will clog the vacuum hose.
• The wall brush: it loosens what adheres to the walls, steps and waterline and brings it down to where it can be vacuumed up. Soft bristles for liner and polyester, firmer ones for coated concrete.
• The manual stick vacuum, connected to the skimmer or vacuum port: simple and economical, it uses the filtration pump. It requires patience and some technique.
• The electric autonomous vacuum, such as Oase Pondovac models: it has its own pump and discharges the extracted water outside. It does not depend on the filtration system, which is valuable for fine deposits or an above-ground pool without a vacuum connection.
• The robot: it moves across the bottom on its own and traps deposits in its own filter. Ideal for regular maintenance, but less suitable for major cleanups.

To connect a manual vacuum properly, see our article on connecting a pool vacuum to the skimmer: an improperly purged hose can cause the pump to lose its prime and waste a lot of time.

The step-by-step method for a clean pool bottom

Here is the order of work we recommend for manual vacuuming. For a standard-size pool, allow thirty minutes to one hour.

1. Let the water rest. Ideally in the morning, before restarting the filtration system, or after stopping the pump for one to two hours. Suspended particles will settle and collect on the bottom.
2. Remove large debris with a net. Leaves, insects, twigs: anything that could clog the hose or the pre-filter basket.
3. Brush the walls, steps, and corners as you move downward, using a slow motion. The aim is to make the deposits fall, not stir up a cloud.
4. Choose the valve position with the pump stopped: “filter” for sand and moderate deposits, “waste” for fine dust and sludge (see below).
5. Purge the hose by filling it completely with water before connecting it, so you do not send air into the pump.
6. Vacuum slowly, in slightly overlapping parallel strips, moving from the shallow end toward the deep end. Moving too quickly stirs up the finest particles, which will settle again behind you.
7. Monitor the pressure gauge and the pre-filter. If the pressure rises significantly, stop, empty the basket, and backwash the filter before continuing.
8. Finish with a backwash if you have vacuumed a significant amount of deposits in the “filter” position, then rinse before returning to filtration.

Filtration or waste: the setting that changes everything

This is the most common mistake. A sand filter correctly retains particles of a certain size, but very fine dust, storm-borne clay, or dead algae particles partly pass through it. If you vacuum these deposits with the valve set to “filter,” you simply send them back through the return jets. The bottom looks clean for an hour, then the film reappears.

In the “waste” position (sometimes called “drain” or “to waste”), the water being vacuumed does not pass through the filter: it goes directly to the drain. The deposit leaves the pool permanently. The trade-off is that you lose water, lowering the water level by a few centimeters for a complete vacuuming. Monitor the level so it does not fall below the skimmer openings, then top up with fresh water, which means checking the pH and disinfectant again.

Self-contained vacuums also discharge the extracted water outside the pool: they effectively work like vacuuming to waste, without using the filtration pump. For an informed choice between sand, glass, or cartridge filters, our article on choosing a sand filter for a pool explains what each medium retains.

Deposits that require more than a quick vacuuming

Mustard algae

It forms a yellow, ochre, or light brown film, often in shaded areas and on north-facing walls. It brushes off easily, which can make it seem like dust, but returns within a few days. It resists normal disinfectant doses better than green algae and hides in accessories, swimsuits, and the inside of pool cleaners. Treatment involves vigorous brushing, a suitable shock treatment, continuous filtration, and cleaning everything that has been soaking in the water. Our article on treating mustard algae explains the procedure in detail.

Precipitated limescale

When pH and alkalinity are too high, especially in hot, hard water, calcium carbonate precipitates. A whitish film can then appear, sometimes with slightly milky water, followed by a rough deposit on the bottom and steps. Vacuuming removes the loose powder, but only bringing the pH back to around 7.2 and maintaining moderate alkalinity prevents precipitation from continuing.

Metal stains

Brown or rust-colored for iron, black or purplish for manganese, greenish or blue-black for copper. They come from well water, mineral-laden top-up water, or equipment corrosion. They cannot be removed with a brush or vacuum. Treat them with a metal sequestrant and, above all, find the source so they do not form again.

Sludge after a storm

Heavy rain can bring soil from the pool surround or nearby flower beds. The bottom then becomes covered with a soft brown layer that clouds the water at the slightest movement. First remove the debris with a skimmer net, let it settle, then vacuum to waste. If the layer is thick and old, it is closer to a sludge problem, discussed in the article suggested below.

Pool robot: what it does well, and its limitations

An electric robot makes routine maintenance much easier. Programmed two or three times a week, it collects sand, dust, and small debris before they accumulate. It does not use the pool's filtration system and retains particles in its own filter, which is often finer than that of a sand filter. For a family that swims a lot, it is a real time-saver.

Its limitations are well known. A large quantity of leaves quickly fills its basket. Thick sludge clogs its filter within minutes. Steps, tight corners, and certain staircases remain poorly covered. And it does nothing against algae or precipitate: it collects some of it, but the cause remains. Also remember to rinse its filter after each cycle and remove it from the water between runs, especially if the pool receives shock treatment.

The robot therefore does not replace the brush and skimmer net. It complements them, and it is all the more effective when the bottom is already clean when it starts working.

Preventing deposits from returning

A bottom that gets dirty very quickly is often a sign of an imbalance somewhere. A few points to check:

• Filtration time. During the season, the most common rule of thumb is to filter for a number of hours per day close to half the water temperature in degrees. This is only a rough guideline, to be adjusted according to pool use and weather.
• Filter condition. Sand that is several years old, clogged, or conversely eroded by preferential channels lets many more particles through. Pressure that no longer drops after backwashing is a warning sign.
• Jet orientation. Return jets directed slightly downward and in the same rotational direction create a current that brings particles toward the skimmers and limits dead spots.
• The tarp or cover. When the pool is not in use, it blocks leaves, dust, and pollen at the source and reduces evaporation.
• The surrounding areas. A clean pool deck, a foot bath, or simply rinsing your feet greatly limits the amount of sand and soil entering the pool.
• Water balance. A stable pH and moderate alkalinity prevent precipitation; correctly dosed disinfectant prevents algae.

Weekly water testing during the season helps anticipate these issues before they become visible on the bottom.

A simple routine, from opening to winterizing

To keep the bottom clean without spending your weekends on it, small regular tasks are better than one major occasional cleaning:

• Several times a week: skim the surface, empty the skimmer baskets, and run the cleaner if you have one.
• Every week: brush the walls and steps, vacuum the bottom, and test the pH and disinfectant.
• After a storm or strong winds: use a leaf net, allow the water to settle, and vacuum to waste if the dust is fine.
• Every month: backwash the filter according to the pressure, and check alkalinity and hardness.
• Before winterizing: vacuum the bottom and brush the walls to prevent an organic layer from decomposing throughout the winter.

In spring, a pool that has been properly winterized is much easier to restart. If you own an above-ground model, our article on cleaning an above-ground pool after winter covers the steps specific to this type of pool.

Also read

When the bottom is covered with a thick, soft layer: How to clean sludge from a pool?.

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Frequently asked questions

— Why does the dust return to the bottom right after vacuuming?

Because it is fine enough to pass through the filter: when vacuumed in “filter” mode, it is sent back into the pool through the return jets. Vacuum these deposits with the valve in “waste” mode, or use an autonomous vacuum that discharges the water outside the pool.

— Should I stop the filtration before using the manual pool vacuum?

You must stop it while changing the position of the multiport valve, never while the pump is running. For vacuuming itself, the pump must be running, since it creates the suction in the hose.

— Is a pool robot enough to remove all deposits?

It handles routine maintenance very well: sand, dust, and small leaves. It is less effective with a large quantity of leaves, thick mud after a storm, or attached algae, which first require a skimmer net, a brush, or water treatment.

— How can I tell whether a stain on the bottom is algae or limescale?

Brush it. Mustard algae lifts into a cloud and then settles again; limescale is rough, does not move, and foams when a drop of vinegar is applied to it outside the water. A brown or black stain that does not come off may indicate metals.

In summary

• Identify the deposit before taking action: sand, fine dust, organic matter, algae, limescale, or metals.
• Let the deposits settle, brush the walls and steps toward the bottom, then vacuum slowly without stirring up a cloud.
• Vacuum large deposits in “filter” mode, fine dust in “waste” mode, then top up the water level.
• Address the cause: sufficient filtration, a filter in good condition, balanced water, and a cover to keep out leaves and dust.
• A robot simplifies routine maintenance, but it does not replace a skimmer net or brush after a storm or algae bloom.

Last updated: October 3, 2026.