Fish make an excellent addition to the garden pond. Here are a few things to remember if you are planning to put fish in your pond.

TYPES

GOLDFISH (Carassius auratus) - Originated in China before 1000 AD. Years of selective breeding have produced the many of varieties goldfish available today. Common goldfish and shubunkins are able to tolerate temperature from 0-32o C   (32-90o F). The fancy varieties of goldfish such as fantails, orandas, and ryukins prefer temperatures from 13-32o C (55-90o C) and therefore are not well suited to spending the winter outdoors. Their maximum size (35-40 cm / 14-16 in) is considerably less than koi, which makes goldfish a better choice if you have a small pond.

KOI (Cyprinis carpio) - Modern koi varieties originated in Japan. There are 14 classifications of colour pattern of koi. Like goldfish, koi can tolerate temperature from 0- 32o C (32-90o F). Small koi are suitable for most ponds, but one must keep in mind that they can reach lengths of 90 cm (36 in)  or more. Even at half this size, koi can cause many problems by constantly stirring up detritus and silt from the bottom, and eating soft new plant growth in a well planted pond.

ORFE   (Leuciscus idus) - Originating in Europe where they are found in lakes and slow moving rivers, the most common type for ponds is the golden form, the golden orfe. These fish are quite active, and grow quite rapidly, but they can be somewhat shy unless they are kept in small schools of 6 or more. The orfe can reach a maximum length of 45-60 cm (18-24"), so it may outgrow small ponds.

MOSQUITO FISH (Gambusia affinis)- The mosquito fish is a small and,  grey fish with a rounded tail and a upward-pointing mouth adapted for feeding at the water's surface. closely resembling  guppies, which belong to the same family. The females don't lay eggs, but are live bearing, producing  broods as large as 60 to 100 young. Females are larger than males reaching length of  7 cm (2.5"), while males reach only 4 cm (1.5").

 STOCKING

It is best to introduce fish into your pond gradually. Add no more than 5 or 6 small fish at one time to allow your pond to achieve a natural balance and prevent an ammonia build up. In established ponds, a maximum stocking density of 1-2 inches of fish per square foot of surface area is acceptable. Here is a general guideline for how many inches of fish you can stock in a filtered pond.

POND VOLUME	TOTAL INCHES OF FISH
1000 gal	110"  eg - 11 ten inch fish, or 22 five inch fish
2000 gal	150"  eg - 15 ten inch fish, or 6 twenty five inch fish
3000 gal	185"   ... and so on
4000 gal	215"
5000 gal	240"
6000 gal	260"

Of course this value can change depending on amount of plants in the pond, whether a biological filter is used, the depth of the pond, and many other factors. When purchasing fish, remember to leave plenty of room for your fish to grow and thrive in your pond.  

FEEDING

The amount of feed and type of food depends on the season, more directly on the water temperature. Being cold blooded, a fishes body temperature depends directly on the water temperature. The warmer the water, the higher the fish's metabolic rate and vice versa. The same is true for the bacteria that live in the fish's digestive system and break down their food. These bacteria become inactive at about 46°F or 8°C, therefore you should not feed your fish when the water temperature drops much below 50°C. Goldfish and Koi have delicate digestive systems, it is best to feed them only what will be eaten in a couple of minutes 3-4 times a day, rather than letting them gorge once a day.