Position
The fish needs some protection against constant direct sunlight;
If the pond has no shading, the water will probably be too warm for the fish in typical South African conditions.
No shading causes excessive algae growth;
If you are going to have water plants, remember that the plants require at least five hours of direct sunlight;
If you do have water plants, the plants provide shade and no alternative shade is required;
Nearby trees provide excellent protection, but can also cause too many leaves in the pond which, if not cleared on a regular basis, cause bad bacteria growth and ultimately bad water conditions;
Care should be taken not to place a pond near any possible poisonous trees or scrubs. (Refer to the list of plants in annexure)
Depth and size
The golden rule is, the bigger the better. It is easier to maintain good water conditions in a big pond, temperature fluctuations are reduced and the fish will in general be healthier, happier and grow faster. There should not be more than 2 fairly large Koi per 1000 litres of water.
The pond must be in proportion to your property;
In respect of depth, the deeper the pond, the better the fish can escape from extreme weather conditions. If it is extremely cold or extremely warm, the fish will go to the deep end;
You can see the Koi so much better in shallow ponds.
In my experience, a deep end of at least 1 metre and a shallow end of at least 500 cm works well for a typical home pond, however, the ideal depth for Koi is 1.5 / 2 metres.
With a smaller, deeper pond, you limit evaporation of water because you limit high temperature and surface area.
To calculate the water volume, you multiply the width, length and depth to get the total volume. A 4 m length x 2 m width x 1 m depth will contain 8000 litres if filled right to the top.
A pond always appears to be at least 20% smaller when it is filled, therefore, always go 20% bigger than what you planned for.