Fish

Take the fish in the plastic bag you purchased them to the pool as quickly as possible.
Float the bag in the water for 15-30 minutes so that the temperature of the water in the bag can adjust to that of the pool.
The top of the bag can be opened and rolled down to form a float that will keep the bag upright. Whilst it is floating slowly introduce several cupfuls of pool water so that the fish can acclimatise to the new water chemistry. This further reduces the possibility of stress.

Now you are the lucky owner of a water garden or backyard pond, you may have always intended that this would be a home for fish. But what fish? 

Apart from ordinary Goldfish, fish that are suitable for average pools of any size include: Red Comets (Sarasas), Shubunkins, Golden Orfe, Tench and Rudd. These will all happily live with one another. More ornamental cold water Goldfish such as fantails and bubble-eyes, find it difficult to compete with other types particularly in the colder months. It is probably best to 'over winter' them in an aquarium indoors if you want to keep them There are always exceptions to the rule and here in one of my first ponds, koi do live in a tolerable harmony with plantssafe and healthy.
Various forms of ‘Fantails’, being slower movers than many pond fish, would find it difficult to compete in winter 
All the above fish are quite content to share their lives with Koi Carp. However serious keepers of Koi Carp need to think in terms of a minimum depth of 3ft - preferably 5 to 6ft - with a proportionately large width and length.
Koi keepers very often have to think in terms of no plants or at least have them protected from the vandalism of the fish. Therefore because of the lack of the biological cycle in which plants form an essential link, and also because of the rapid metabolism of these potentially very large fish, a sophisticated filter system is essential in keeping Koi. They are not fish of extremely cold water and find our winters stressful to say the least. When fish are stressed, then they are open to disease and parasites, and since Koi are such expensive fish, the only way to keep them with confidence is to employ all the quirks of pool design and filter technology at your disposal. (See all the recent biofiltration articles)

But first, before you even think of introducing any fish to your pond.......

Even low levels of chlorine and chloramines can be extremely toxic to fish. Chlorine is a very powerful oxidizing agent and it is toxic to fish at concentrations of less than 0.05 mg/L. Water used for fish culture should not contain any residual chlorine to be considered safe. Chlorine and chloramines cause gill damage which eventually results in gill lesions. These lesions will eventually thicken the gill filaments leaving the fish unable to utilize oxygen and release carbon dioxide, thus resulting in fish death. Many of the 'Chlorine removers' (in liquid-form) from your aquatic store  will work to break the chlorine and chloramine bonds (chlorine + ammonia = chloramine) and reduce the chlorine and ammonia down to non-toxic levels.

Many of these chlorine removers contain sodium thiosulfate, which is widely used to break down chlorine and chloramines and is considered non-toxic to fish. Sodium thiosulfate was chosen because it provides a very effective solution for use in water gardens and ponds because of its ability to break the chlorine bond and chemically remove the chlorine after a short working time. Aeration of the water (which the majority of water gardens have present) will allow the ammonia, which was released from the chlorine bond, to escape as a gas over a period of one to two hours. Any remaining ammonia will be filtered out through flow through filters which most often contain activated carbon or some type of filter material.

Best used for regular water changes and when filling a new pond. Manufacturer recommends not putting in any fish or plant life until a day after treatment to be safe. This should give any residual ammonia enough time to escape or filter out.