A garden pond can be left to its own devices as a miniature nature reserve, but will be a much more attractive feature if it is stocked with aquatic plants and ornamental fish. Get the natural balance right and the pond will be virtually self-sustaining, needing only occasional attention to keep it is good condition.
First you need plants and there are several distinct groups of plants you can grow in or around a garden pond. These are the main ones:
Oxygenators are essential plants for keeping the pond healthy. Some are rooted, but most 
simply float in the water, absorbing carbon dioxide and releasing oxygen as they grow.
Water Lilies are planted in containers that sit on the bottom of the pond and produce leaves and stunning flowers on the surface in the summer.
Marginals are planted in containers set in shallow water on shelves around the pond margins. This group includes various species of Iris, Flag, Marsh Marigold and the Arrow Head.
Floaters such as Fairy Moss, Greater Bladderwort and Water Soldier are flowering plants that simply float on the pond. The Water Soldier sinks to the bottom in winter.
Marsh Plants, such as Bullrushes, can be planted at the edges of the pond, if the liner is extended and filled with soil to create waterlogged bog conditions.
The humble Goldfish, bred by Chinese and Japanese fish-keepers from a dull brown wild species, is the most common fish kept in garden ponds. Other species you could consider include Tench and Golden Orfe. The Golden Orfe with its gold and black markings, is by contrast, an active surface feeder and an excellent display fish. However, it grows quite large so is not suitable for small ponds.
If you have a larger pond you can consider keeping Koi. These are an ornamental species of carp, much prized by the Japanese for their exotic colouring and marking. They come in single colour, two colour and multi-coloured varieties, further distinguished by their scale development.
To assess how many fish your pond will support, estimate its surface area and allow 60sq cm 
of surface for every 1 cm of fish (equivalent to 24 sq. inches per inch of fish). Since fish grow and breed, it is best to start off with around one-third of the theoretical maximum number. For example, a 1.8 x 1.2m (6ft x 4ft) pond will, in theory, support 360cm (144ins) of fish, equivalent to 36 individuals averaging 10cm (4ins)long, but, in practice, you should have only about 12.
Golden Rule – Your pond needs to have part of the area at least 60cm deep for goldfish and 1.5m or more for Koi if they are to be left over winter safely in the pond.
The biggest enemies of your pond and its inhabitants are algae in summer, fallen leaves in autumn and ice in winter.
- To discourage algal growth, remove anything shading the water, especially overgrown water lilies and oxygenators. Treat the water with a proprietary algicide. Also see “Filters and Purifiers”.
- In hot weather, top the pond up occasionally to replace water lost by evaporation. If you have a fountain or waterfall, run it at intervals to help aerate the pond.
- Net the pond in Autumn to prevent leaves drifting into the water and decomposing, reducing the oxygen level. Clear the netting of leaves at regular intervals.
- Place a small floating ball in the pond to keep part of the surface clear of ice during frosty weather, so that gases in the water can disperse. Do not break ice by force, you may stun and injure the fish which will lie happily dormant in near-freezing temperatures.
- source:Hints and Things
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