The pre-formed pond is usually made from plastic, fiberglass or a PVC or rubber composition. These come in a variety of shapes and sizes from which you may choose. They are also very rigid in nature unlike a liner.
If you are planning a formal pond that is circular, oblong or square, choose a pre-form over any other type of construction material. It will be true to form and the easiest to install.
Informal pre-form liners are a different situation altogether. Manufacturers make them for the shape and visual appearance to sell product and do not necessarily adhere to water gardening standards in the design. Many have useless plant shelves that are to narrow and shallow for most marginal plants and the depths vary greatly. Opt for the standard shapes like kidney and go for the deepest pre-form you can afford. Usually 18" is sufficient for most applications, however a 2-foot depth is preferred in colder climates.
INSTALLING A PREFORM
Plan on from the beginning digging a larger hole than the size and shape of your pre-form pond. This way you can have a level bottom, make adjustments and backfill the sides for added strength and support.
Marking it All Out:
Before actually starting to dig, drive stakes into the ground at regular spacing around the edge of the site. Do this by laying the pre-form upside down in the excavation area. Place the stakes about 1 foot away from the farthest point both length and width. Level the tops of each peg or stake using a 2x4 and level. The one that projects most from the ground will be your low point and the soil should be removed from the entire area so that the distance between the top of the stake and the ground is equal.
Digging it All Out:
Now you can start to dig the rectangle or square hole that you have just laid out. It will be much easier than trying to dig out the exact shape of the pre-form pond. The depth of the home should be 2" deeper than the actual depth of the pre-form liner. This area you will backfill with sand to give the pond a cushioned firm platform on which to rest.
Don't plan on using the soil you remove to backfill the pond. Most of it will not be the right consistency and should be discarded. If the soil, however, is easily formed and flows around the pond then you may consider using it. Otherwise use topsoil to fill in the area.
Installing It All:
Once you have dug the hole, place the sand on the bottom, misting it with the hose and tamping the surface compactly. You must be sure that the pre-form is supported under all sections of the shape to ensure it will remain level and resist damage if stepping on the edges. You may also use pea gravel as the last layer to insure a firm surface for your pond. Constantly check the levelness of your pond to be sure you can adjust it prior to completion. Now you can feel free to start filling your pond. As you do, once again, keep checking the levelness of the pond. Leave the pond sitting overnight to let the waters weight seat the pond. You can then decide what edging you will use and finish the pond accordingly.
