fish
With the availability of exotic fish from habitats all around the world, it has become even more important to create an environment suitable for mixed populations of fish. The basic requirements for keeping fish remain the same - clear, well-oxygenated water, varied depths, adequate shelter, shade and food.
Overstocking a pond is a sure route to failure. Avoid buying more fish (or larger fish) than your pond will support. Fish often take care of overstocking by themselves, by eating their own spawn or not spawning at all.
The number of fish your pond will support depends on the surface area, (because it is this factor that chiefly determines how much oxygen will be available to them), and the length of the fish when mature (because their size determines their oxygen needs). An area of 1ft x 1ft will support a fish 2 inches long. While an area three times as big will support three such small fish or allocated to one 6 inch fish.
When you start out, buy small fish of less expensive breeds. This will allow you to get comfortable with the routine of fish care, to observe any problems with your water supply or potential predators, and to see how your fish adjust to their new home.