One of the simplest ways to install a pond in your garden is to use a rigid, prefabricated liner. They are a particularly good choice for small to medium sized ponds in formal shapes such as rectangles, squares and L-shapes. Irregularly shaped curved pools may also be found, however, these are a little trickier to install than the straight edged versions; the more complicated the contours, the harder it is to fit the pond correctly into the ground.

Preformed liners come with a variety of features already built in, such as:

• shelves for planting baskets,
• areas to site pumps,
• marsh borders - shallow areas around the edges of the pond that offer amphibians easy access to and from the pond and can be planted up with bog plants.

Preformed pools are available in rigid plastic and fibreglass; both are very strong and should last about 15 years. Choose a pool that is at least 45 cm (18 in) deep, especially if you intend to stock fish. When ordering a liner, ask if the shop will deliver - you may find that a 500 gallon pond will not fit easily into the boot of your car! Check the shell for damage or faults before installing.

Marking the Site

Level the site and clear away any weeds or stones. Invert symmetrical moulds such as circles, squares and rectangles (i.e. those that offer the same profile upside down as they do the right way up) and mark the shape using pegs and string. Stand an asymmetrical mould the right way up and then support it with bricks to keep it level and upright. Mark its outline using a number of bamboo canes pushed into the ground around its perimeter. Wind a piece of rope or string around the canes to indicate the precise shape. Make a second line about 30 cm (12 in) further out to show the area to be excavated.

Digging the Hole

Following the excavation lines, dig the hole to a depth of 5 cm (2 in) deeper than the marginal shelves. The sides should slope slightly towards the centre of the pond. Remove any large stones or roots as you go along. Save the topsoil for use around the garden, but keep the subsoil to backfill the hole.

Place the liner in the hole and press it down firmly so that the deep water area at its base leaves a clear mark in the soil. Dig out this area, again making sure that the sides slope slightly. Dig to a depth and width to allow for a 5 cm (2 in) layer of cushioning material.

As you dig, remove any sharp stones, tree roots or debris. You must also keep checking that each layer is absolutely level - use a plank and spirit level to check.

Lining the Hole

Tamp the soil down firmly and then add a 5 cm (2 in) cushioning layer of damp sand, polyester matting or fibreglass loft insulation material.

Check that each layer of the pond is absolutely level and that the depth of the hole is correct. Lower the mould into place and check with a spirit level and plank that it is set level.

Filling the Pond

Gently add about 10 cm (4 in of water) and then backfill the hole around the liner to reach this depth. Tamp the soil down and check that the liner is level. Keep repeating this process of adding water, backfilling the hole and levelling the pond. Once full, leave the liner in place for at least 24 hours before adding the edging.

Finishing the Edge

Firm the earth around the pond and line the edges with paving stones bedded on mortar or turves. Paving stones should overhang the edge of the pond by about 5 cm (2 in) to help hide the liner from view.