Build a pool without regretting it 3 years from now: the 10 mistakes we see (far too) often

Every spring, we see owners coming in who want to «redo their pond», built two or three years earlier. The water is always green, leaks cannot be found, fish die in summer, and the edges collapse. Almost always, the problem goes back to a decision made when the pond was dug. A pond is built once, but lived with for twenty years: a few centimeters of depth or a few euros saved on the liner at the outset are paid for throughout its life. Here are the ten mistakes we encounter most often, and how to avoid them.

The 10 mistakes

— 1. A pond that is too small

This is the number one regret. The larger the volume of water, the more stable it is: its temperature varies less, pollutants are diluted, and biological balance is established more easily. A 1 m³ pond is much harder to keep clear than a 10 m³ pond. And the desire to add fish always comes. If you are hesitating between two sizes, choose the larger one.

— 2. Not deep enough

Less than 60 cm deep, the water heats up too quickly in summer and freezes too deeply in winter. Plan for at least 80 cm to 1 m in the deepest area for goldfish, and 1.20 m to 1.50 m or more for koi. This is also the best protection against herons.

— 3. Poorly designed shelves

Planting shelves must be at the right depth (10 to 20 cm and 30 to 40 cm below the water level) and wide enough to hold baskets. But a pond made only of shallow shelves becomes a buffet for herons and a source of hot water in summer. Keep steep sides in part of the pond.

— 4. A low-quality liner

A thin PVC liner or an agricultural tarp costs less, but hardens in the sun, cracks in freezing weather, and tears easily. EPDM, a synthetic rubber about 1 mm thick, stays flexible in very cold weather, resists UV rays, and can be repaired. Its lifespan is measured in decades. Over the life of the pond, it is by far the most economical choice.

— 5. No underlay beneath the liner

A stone, a root, or a buried piece of glass: a single pressure point is enough to puncture the liner years later. A thick geotextile under the liner, and ideally a second layer on top in areas where stones are placed, is inexpensive insurance.

— 6. Uneven edges

A difference of just a few centimeters between two edges, and the liner sticks out on one side while the water overflows on the other. Check the level all the way around before laying the liner, using a laser level or a board and spirit level.

— 7. Forgetting the capillary barrier

This is the number one cause of “untraceable leaks.” If the soil or roots of waterside plants touch the water over the edge of the liner, they draw it away by capillary action, day and night. The edge of the liner must rise vertically above the water level, behind the stones or edging, to prevent any contact between the water and the garden soil.

— 8. An undersized filtration system

Manufacturers often indicate two capacities: without fish and with fish. The latter is what matters, and for koi, you need to allow even more capacity. An undersized filter requires constant cleaning and allows the water quality to deteriorate as soon as the first hot weather arrives.

— 9. Nothing planned for water and electricity

No buried conduit for cables, no residual-current protection, no overflow to drain stormwater, and no wall penetration for a future bottom drain. All of this can be planned in a few hours during construction, but becomes a major project once the pond is full. Electrical installations near water must comply with current standards and should ideally be carried out by an electrician.

— 10. A poor location

Under a large tree, the pond fills with leaves. In full sun with no shade, it heats up and turns green. At the lowest point of the site, it receives runoff laden with fertilizer and mud. Ideally: five to six hours of sunlight, some afternoon shade, a slight elevation above the surrounding ground, and a view from the house.

Calculating your pond liner correctly

The safest formula: length = maximum pond length + 2 × maximum depth + 2 × 50 cm of allowance, and likewise for the width. The 50 cm on each side is used to raise the capillary barrier and anchor the liner. Example: a 4 × 3 m pond, 1 m deep, requires a liner measuring approximately 7 × 6 m.

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Frequently asked questions

— What is the minimum depth for a pond with fish?

At least 80 cm in the deepest area for goldfish in our regions, and 1.20 m to 1.50 m for koi. Below that, the water gets too warm in summer and freezes too much in winter.

— EPDM or PVC liner?

EPDM, without hesitation, for a pond you want to keep for a long time: more flexible, more resistant to UV rays and frost, and easier to install without wrinkles. PVC may be suitable for a small temporary pond.

— How long does it take to build a pond?

A medium-sized garden pond can generally be built in one to three weekends, including excavation. Then allow several weeks before introducing fish.

— Do you need a permit to build a pond?

It depends on the area, depth and local planning regulations. Check with your municipality before digging, especially for a large pond.

In summary

• Larger and deeper than you imagine.
• EPDM over underlay, level edges, capillary barrier.
• Filtration "with fish", or more.
• Electrical systems, overflow and wall penetrations planned from the outset.

Last updated: September 28, 2026.