Preformed pond or a liner?

A preformed pond is a rigid shell. A liner is a sheet you shape in a hole you dug. The choice is about the edge and the depth, not about which one looks more expensive in a catalogue.

What you give up with a shell

The RHS wildlife guide says preformed plastic or fibreglass ponds do not usually have a long shallow slope. Wildlife needs that slope, or a ramp you add afterwards, on the inside and on the outside if the shell sits above ground. A beach of gravel on a liner is easier to shape than a beach fought out of a moulded step.

Shells are honest about their volume. The litres stamped on the shell are the litres you have, as long as you set it level. A liner pond's volume depends on how you dug it, which is why the planner asks for average depth.

What you give up with a liner

You have to make the hole, protect the sheet, and anchor the edge. The size sum is on the liner page. A liner also lets you build the 20-degree side slopes the RHS suggests so ice has room to expand. A vertical preformed wall does not.

Butyl, in the RHS's words, is the easy flexible liner for a natural outline. EPDM rubber sheet is the type commonly sold for the same job. Do not decide from a product title alone: read the thickness and whether the seller expects underlay. Underlay is not a liner.

A practical split

• Choose a shell if the pond is small, you want a known volume, and you are willing to add a ramp for wildlife.
• Choose a liner if you want a beach, a planting shelf in a place you choose, or a shape that is not a mould.
• Do not choose either yet if young children use the garden and you have not read the safety page. A shell is not a safety device.

Setting a shell

The ground under a shell has to support it without a void. A hollow under the base will crack or collapse once it is full of water. Bed it on sand, check the rim is level, and backfill evenly. The RHS wants the surrounding ground level to begin with. A shell set out of level shows a white rim on one side and overflows on the other.

If the shell is raised, wildlife cannot reach the water unless you build a ramp outside as well as in. The RHS says that specifically for container ponds that are not sunk.

Are preformed ponds deep enough for fish?

Read the shell. The RHS wants at least 38 cm somewhere, ideally more, and at least 3.5 square metres of surface if you are keeping fish. Many small shells fail the surface test even when the depth looks plausible. A wildlife pot can be shallower because it is not trying to be a fish pond.

Can I add a liner to a cracked shell?

Sometimes people do, as a patch. It is usually cleaner to treat it as a new liner pond and dig the shape you actually want. Measure before you buy the sheet.