Gravity-fed or pumped filtration: which system should you choose for your pond?

Should the pump be placed in the pond and send water to the filter, or should water flow by gravity to a buried filter, with the pump returning only clean water? A gravity-fed system filters better and often uses less energy, but the decision must be made during construction; a pumped system is simpler and cheaper initially, but it quickly reaches its limits with koi. Here is how to decide, with figures and installation guidelines to support your choice.

The two main filtration architectures

Whatever the equipment, a filtered pond circulates its water in a loop: it leaves the pond, passes through a filter, then returns to it. The entire difference between the two systems comes down to where the pump is located, and therefore what passes through it.

• In a pumped system, the pump is in the pond. It draws in dirty water, including debris, and pushes it toward a filter placed beside the pond, above the water level. The filtered water flows back down into the pond by gravity, often via a waterfall or stream.
• In a gravity-fed system, water leaves the pond through one or more bottom drains and a skimmer connected to large pipes. It flows by itself to the filter, whose water level is the same as that of the pond. The pump is located after the filter and only returns water that has already been filtered.

On paper, the difference seems slight. In reality, it affects filtration quality, installation cost, electricity consumption, maintenance, and even how the pond is built. This is not a detail to adjust afterward: the choice between gravity-fed and pumped should ideally be made before digging, because a gravity-fed system requires bottom drains and pipes to be installed during construction.

Pumped system: principles, advantages, limitations

This is the most common setup in ornamental ponds. A submersible pump, placed on a slab or slightly raised support, feeds a multi-chamber filter, a pressure filter, or a box filter set on the ground. The return is via overflow.

What argues for pumped systems:

• Simple installation, possible on an existing pond without emptying it.
• A lower initial budget: no bottom drain and little excavation work.
• Great flexibility in filter placement, as long as it remains above the water level.
• Easy-access maintenance: the pump can be removed from the pond in a few minutes.

Its limitations, especially with koi:

• The pump draws in the debris and partially grinds it. Whole leaves and droppings become fine particles that are much harder to retain and cloud the water.
• It only draws in what passes nearby. Sludge accumulates elsewhere on the bottom and must be suctioned out regularly.
• It must push the water to the top of the filter, then overcome the head losses in the pipes and elbows. At the same flow rate, it consumes more than a return pump in a gravity-fed system.
• A solids-handling pump has a strainer with large openings: small fish and fry can approach it.

A pumped system remains perfectly suitable for a decorative pond of a few cubic meters, moderately stocked with goldfish or orfe, with plenty of plants. For a large koi pond, it quickly reaches its limits.

Gravity-fed system: principle, advantages, limitations

In a gravity-fed system, the pond is equipped during construction with one or more bottom drains at the lowest point, and a wall skimmer that collects surface water. Each intake is connected by its own pipe, generally 110 mm pressure-rated PVC, to the filter inlet. The filter is buried so that its water level matches that of the pond. A pump placed in the last chamber returns clean water to the pond, often through return jets that create a circular current.

What argues for gravity-fed systems:

• Debris reaches the prefilter whole (drum, screen, vortex), making mechanical filtration much more effective.
• The bottom of the pond is continuously suctioned, so sludge hardly accumulates anymore.
• The pump only handles filtered water: less wear and no strainer clogging.
• It has almost no height to overcome, allowing the use of economical pumps designed for this type of installation.
• The pond remains uncluttered: no equipment is visible in the water.

Its constraints:

• It must be planned during the design stage: bottom drains, wall penetrations, and trenches are installed before waterproofing.
• The filter must be buried or installed in a lower technical room, which requires excavation work.
• The initial budget is higher: bottom drains, large-diameter piping, valves, and a suitable filter.
• A grading error can be costly: a filter installed too high will not fill, while one installed too low will overflow at the slightest shower.

For installing a bottom drain, we refer you to our article on bottom drains.

The communicating vessels principle and the filter level

Gravity flow is based on a simple physical principle: two volumes of water connected by a pipe tend toward the same level. If water is removed from the filter (this is the pump's role), the level there drops slightly, and the pond water flows in to compensate. The higher the pumped flow rate, the greater the difference in level between the pond and the filter, because the water must overcome the resistance of the pipes, elbows and prefilters.

In practice, this difference remains around a few centimeters in a properly sized installation. If it exceeds ten centimeters or so, this indicates that the pipes are too long, too narrow, obstructed, or that the requested flow rate is too high.

Three elevation rules to follow:

1. The filter rim must be above the pond's maximum level. When the pump is stopped, the levels equalize. If the pond rises after heavy rain, the filter level rises too. Leave a comfortable margin above the pond's overflow.
2. The filter's operating level must keep the pump submerged. During operation, the level drops in the last chamber: the pump must remain underwater with a safety margin, otherwise it will draw in air.
3. The filter inlets must remain underwater. A gravity-fed inlet that opens into the air draws air into the pipe and greatly reduces the flow rate.

The pond overflow also deserves special attention. A well-positioned overflow connected to a drain protects both the pond and the equipment room. Without one, a storm can flood the filter room.

Pipe diameters and gravity flow rates

With gravity flow, water is not pushed: it flows under the effect of a very small difference in level. The pipes must therefore be wide, short and as straight as possible. The following table gives approximate values commonly used for a pond installation. They vary depending on the length of the pipes, the number of elbows and the accepted difference in level.

For a 30 m³ pond that you want to filter approximately once every hour and a half, you therefore need at least two or three 110 mm inlets, for example two bottom drains and a skimmer. You cannot compensate for an insufficient diameter with a more powerful pump: you simply increase the difference in water level until the system loses its prime.

A few details make all the difference over time:

• 45° elbows rather than 90° elbows, which restrict the water less and clog less easily.
• A slight, consistent slope toward the filter, with no low points where sludge can stagnate.
• A knife gate valve on each line, at the filter inlet. It allows you to isolate a pipe, adjust the flow distribution, and flush the lines: close it for a few minutes, then open it suddenly to dislodge deposits.
• Glued pressure-rated PVC, not thin drainage pipe, which deforms under the weight of the soil.

Electricity consumption and cost over 10 years

The pump generally runs all year, day and night. Over such a period, a difference of a few dozen watts adds up significantly. The principle is simple: a pump consumes more the higher it has to lift the water and the longer the pipes it must overcome. In a pump-fed system, it must reach the top of the filter. In a gravity-fed system, it only lifts the water a few dozen centimetres to the return jets.

Let's take a numerical example, with assumptions to adapt to your situation:

This calculation does not mean that a gravity-fed system necessarily consumes half as much: it all depends on the pumps being compared, the target flow rate, and the delivery head. It does show, however, that an apparently modest difference in power represents, over ten years, an amount in the same order as the price of a good filter. The power draw is shown on the rating plate and in the technical specifications of each pump: that is what must be compared, at the actual operating point, rather than the advertised maximum flow rate at zero head.

Gravity-fed systems also have hidden costs, but they are mainly upfront costs: bottom drains, piping, valves, earthworks, and possibly a technical room. Pump-fed systems have higher operating costs and require more frequent maintenance (strainer cleaning, sludge removal). Our guide on choosing your filtration pump explains how to read flow-rate curves.

Can you switch from a pumped system to a gravity-fed system?

Yes, but you need to know what you are taking on. A complete conversion requires draining the pond, installing one or more bottom drains in the existing lining, digging trenches to the filter location, and then burying the filter at the correct level. On a liner or EPDM pond, installing a drain on an existing membrane is delicate: a suitable clamping flange and a clean surface are required, otherwise a leak will develop at the lowest point of the pond, the worst possible location.

There is an intermediate solution that is often more reasonable: the pumping chamber. The bottom drains and skimmer feed a small tank buried next to the pond by gravity, with the pump located inside it. The pump then sends the water back to the existing filter, installed above water level. This gives you bottom suction and keeps the pump out of the pond, without having to bury the filter. However, you lose the benefit of intact debris, since the pump still passes through it.

Another possible step is to add a gravity-fed pre-filter, such as a drum filter or a screen, between the pumping chamber and the filter. Our drum filter guide explains how to size it.

Common case: a 15 m³ pond dug ten years ago, filtered by a submersible pump and a foam filter, now houses fully grown koi. The water is turning green and sludge is building up. Rather than rebuilding everything, a spring draining is used as an opportunity to install a bottom drain and a pumping chamber. The existing filter is kept initially, then replaced with a gravity-fed model when the budget allows.

Which system for your pond (table)

One final criterion that is rarely mentioned: the amount of time you are willing to devote to maintenance. A well-designed gravity-fed system with an automatic prefilter reduces maintenance to just a few checks. A pump-fed system requires more attention: cleaning the strainer, rinsing the foams, vacuuming the bottom.

Also read

Before starting with a gravity-fed system: Why is a bottom drain important for your garden pond and how do you install it?.

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Frequently asked questions

— Is a gravity-fed system always better than a pump-fed system?

For a koi pond or a large volume, it filters better and puts less strain on the pump. For a small, sparsely populated ornamental pond, a properly sized pump-fed system is more than sufficient and costs less to install.

— What pipe diameter is needed for a bottom drain?

110 mm is the standard. A flow rate of around 8 to 10 m³/h per 110 mm pipe is generally used; above that, additional bottom drains are added or 160 mm piping is used.

— At what height should a gravity-fed filter be installed?

Its water level must match that of the pond, and its edges must extend above the pond's maximum level with a margin, to prevent overflowing after heavy rain.

— Can a pump-fed pond be converted to gravity-fed without emptying it?

No: installing a bottom drain requires emptying the pond and working on the waterproofing. A pump chamber is often a good compromise.

In summary

• Pump-fed: pump in the pond, filter above water level, simple and economical installation.
• Gravity-fed: bottom drains and skimmer, filter buried at pond level, pump after the filter.
• Communicating vessels require precise levels and large pipes, generally 110 mm.
• Over ten years, the difference in consumption between two pumps can equal the price of a filter.
• A pump chamber is the most common compromise for improving an existing pond.

Last updated: October 3, 2026.