Build a cement pond: foundation, masonry, plastering, and safe filling with water for fish

A cement pond is often a formal pond: a crisp rectangle in the middle of a patio, a water mirror edged with blue stone, or a raised pond where you can sit. No liner creates such sharp angles. In return, the project is more demanding and, above all, fresh cement significantly changes the water chemistry. A successful cement pond relies on three things: a structure that does not crack, a suitable waterproofing coating, and patient filling while monitoring the pH.

Why cement alone is not enough

Concrete and mortar are porous and almost always eventually develop microcracks due to drying shrinkage, frost, and minor ground settlement. An untreated cement pond therefore slowly loses water, sometimes as early as the first winter. Hence the idea of separating the roles: the masonry provides the shape and withstands water pressure, while a dedicated coating ensures watertightness. For the latter, there are two options: a waterproofing render or coating designed for ponds, or a flexible liner installed inside the masonry.

Construction steps

1. Earthworks: excavate to the desired shape, adding the thickness of the foundation and walls.
2. Foundation: a perfectly level reinforced concrete slab, poured over compacted hardcore. It is what prevents the pond from deforming.
3. Walls: concrete formwork blocks laid on the slab's starter bars, reinforced vertically and horizontally, then filled with concrete. Simply mortared hollow concrete blocks do not withstand water pressure well.
4. Penetrations: plan the pipe openings (drain, skimmer, filter return) during the masonry work, using suitable penetration fittings.
5. Plaster: carefully applied cement plaster, with rounded cove corners between the bottom and walls, making it easier to apply the coating and limiting cracks.
6. Waterproofing: multi-coat pond coating, or liner with protective underlay.

For a deep, large-surface, or raised pond, have a professional check the reinforcement: the pressure of the water on a high wall is considerable, and a wall that gives way cannot be repaired.

Coating or liner: which should you choose?

Apply the coating to a clean, dust-free, slightly damp surface, according to the manufacturer's instructions: the number of coats, time between coats, and application temperatures must be followed precisely. A suitable brush helps work the product into the pores and corners.

Filling the pond: the pH trap

Fresh concrete releases lime into the water, making it highly alkaline. An excessively high pH is dangerous for fish: it damages their gills and mucus, and increases the toxic proportion of ammonia present in the water. This is a well-known mechanism in water quality management for fish-farming ponds. Never put fish in a new concrete pond before measuring a stable pH, close to that of your filling water, for several consecutive days.

The process is simple. First, let the concrete and coating cure for the recommended time, generally several weeks. Fill the pond, let it sit for a few days, measure the pH, drain and rinse. Repeat until the reading no longer changes. A suitable waterproofing coating greatly reduces this leaching, but testing is still essential. Plants can be installed earlier; fish come last.

Mistakes that cause a concrete pond to crack

A slab that is too thin or unreinforced and follows ground movement. Hollow concrete block walls without reinforcement. Coating applied in full sunlight or freezing weather, which dries too quickly and cracks. Sharp corners between the floor and walls. And in winter, a shallow pond with vertical walls where expanding ice presses against the masonry: in this case, a compressible floating object or a de-icer limits the pressure. A common case: a fine crack appearing along a corner during the first winter, which could have been avoided with a rounded fillet and a reinforced wall.

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For details on the waterproofing stage: How to waterproof a masonry concrete pond?.

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Frequently asked questions

— Is a cement pond watertight without treatment?

Not reliably. Concrete is porous and develops microcracks over time and with frost. You need a waterproofing coating suitable for ponds or a liner installed inside.

— How long should you wait before filling a cement pond?

Observe the curing time for the concrete and coating specified by the manufacturers, generally several weeks. Rinsing and pH monitoring come next.

— Why does the pH rise in a new cement pond?

Cement releases lime, making the water highly alkaline. Rinse and measure until you obtain a stable pH close to that of the filling water before introducing fish.

— Concrete blocks or reinforced concrete blocks for a pond?

Reinforced concrete blocks filled with concrete, laid on a reinforced slab. Simple hollow concrete blocks bonded with mortar do not withstand water pressure well.

In summary

• The masonry provides the shape; a dedicated coating or liner provides watertightness.
• Level reinforced slab and reinforced concrete block walls.
• Rounded fillets at the junction between the floor and walls.
• Observe the manufacturers' curing times.
• Rinse and measure the pH until stable before introducing fish.

Last updated: September 29, 2026.