What should you put at the bottom of a preformed pond: pebbles, sand, potting soil, or nothing at all?

You have just installed a 500-liter preformed pond, and the smooth black bottom looks too bare. Round pebbles, white sand, good garden soil for the plants? The question is legitimate, and the answer goes against intuition. In a small preformed pond, the best bottom is an almost bare bottom: no potting soil, no layer of sand, and at most a few round pebbles where they serve a purpose, with the plants remaining in baskets filled with nutrient-poor aquatic substrate. Here's why.

Pebbles, sand, potting soil: the comparison

Why potting soil is the worst idea

Garden soil and horticultural potting soil are rich in organic matter, nitrogen, and phosphorus. Underwater, without oxygen, this matter decomposes slowly and continuously releases its nutrients. In a 500-liter pond, that is enough to keep the water green all summer. The potting soil also partly floats, clouds the water every time a fish swims by, and clogs the pump. Aquatic plants do not need it: they draw much of their nourishment directly from the water.

Why layers of sand and gravel cause problems

In a large natural pond, a mineral bottom has its place. In a small preformed pond, it quickly becomes a trap: leaves, droppings, and leftover food slip between the grains and stones. There, without water circulation, they rot without oxygen. After a year or two, disturbing the bottom releases a black cloud and a rotten-egg smell. And a pond vacuum cannot separate sludge from gravel.

The right method: planting in baskets

1. Choose perforated baskets suited to the size of the plants.
2. Line them with fabric such as jute or felt so that the substrate does not escape.
3. Fill with low-nutrient aquatic soil, not horticultural potting soil.
4. Plant, then cover the surface with 2 to 3 cm of washed gravel: it prevents the substrate from rising and the fish from digging.
5. Place the baskets on the shelves of the preformed pond, at the depth indicated for each plant.

Add submerged oxygenating plants such as hornwort, which do not even need substrate: they float midwater and absorb nutrients directly.

Make the bottom look more natural without cluttering it

• A few round pebbles arranged in small groups, with no sharp edges that could injure the fish.
• A beautiful stone or a well-washed submerged root as a focal point.
• The biofilm: within a few weeks, a thin, natural brown-green film covers the black bottom and makes it look much less artificial. Do not brush it off; it harbors beneficial bacteria.
• Plants that, as they grow, largely conceal the shelves.

Maintaining the bottom of a small pond

Remove fallen leaves with a net before they sink, especially in autumn. Once or twice a year, vacuum deposits from the bottom. Do not overfeed the fish: in 500 liters, they should remain few in number and small in size. Koi, in particular, have no place in a pond of this volume.

Read also

To complete the installation: Which filtration system should you choose for a 500-liter pond?

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Frequently asked questions

— Should you put sand at the bottom of a preformed pond?

No. A layer of sand traps sludge and becomes impossible to clean. A bare bottom or a few pebbles are sufficient.

— Can you put soil at the bottom of a pond?

No, it releases nutrients that promote algae and cloud the water. Use poor aquatic soil in baskets.

— Are pebbles useful at the bottom of the pond?

A few rounded pebbles are decorative and provide shelter for bacteria, but a layer across the entire bottom retains sludge.

— How can you make a black bottom look less artificial?

Let the natural biofilm establish itself, add a few pebbles or a stone, and let the plants hide the shelves.

In summary

• No potting soil or garden soil at the bottom of the pond.
• No layer of sand or gravel across the entire bottom.
• Plants in baskets, poor aquatic soil covered with gravel.
• Bare bottom, a few rounded pebbles, and the biofilm will do the rest.

Last updated: September 28, 2026.