The surface area should be large enough for plenty of oxygen to dissolve in the water, which will keep fish happy. Of course, if you're keen on stocking Koi, and big ones at that, you'll have to design the pond with them in mind. So take advice first before installing a pond which may turn out to be unsuitable.

It's also important to have some method of moving water around during still summer days to increase the oxygen content of the water. The sound of moving water is pleasant too, and adds to the pleasures of a pond.

One simple and attractive way to move the water around is with a fountain. But do check the spread of the fountain, and make sure that the pond liner is large enough to catch the returning water, otherwise the pond level will drop dramatically. Although topping up a pond is easy, making sure that the water quality is suitable is not so simple. Tap water contains various impurities which can be toxic to fish and encourage algal growth.

An alternative to a fountain is a waterfall. These are straightforward to install with pre-formed liners, but it's best to incorporate them at the same time as the main pond liner. Most liner manufacturers make waterfall sections to match their range.