Before discussing all the ins and outs of keeping fancy goldfish in a pond we need to determine what makes a good pond for goldfish.
Depth is the first thing to consider. Making the pond deep enough for the fish to survive a cold winter is obviously very important. Water is at its most dense at 4ºC which means that will be the temperature at the very bottom of a well designed pond regardless of the colder temperatures above.
Water will stratify (form in to layers) of different temperatures with 4ºC being the lowest layer even when the water above is frozen solid. 4ºC is a safe and comfortable temperature for goldfish and most other temperate fish to overwinter at.
In order for the water to form the different temperature layers it has to be deep enough, a very shallow pond could freeze solid all the way down to the bottom, this would kill any fish, even if the ice didn't quite reach the bottom of the pond the water would be near to 0ºC and may harm the fishes chances of making it through the winter.
I would recommend that the pond should be a minimum of 75cm (30 inches) deep if fish are going to be kept in it. Such a pond is unlikely to freeze all the way through in most parts of the UK.
Filtration is often overlooked because most novice pond builders compare their small pond to a natural pond and as we all know - natural ponds don't have filters. But a garden pond full of goldfish which are fed every day is a long way off being a natural pond and even the humble goldfish pond DOES need a filter. Also if the water ends up looking like pea soup you won't be able to see the fish so what would be the point in having them?
The filter should comprise of different sections, a coarse filter, biological filter and a fine filter, it should be gravity fed so that the pump can be placed after the fine filter this will prevent the pump from ever being blocked up because it is only ever pumping clean water.
From the pump the water should pass through a UV steriliser which will prevent the water from going green due to free floating algae. You will notice that I have left the UV steriliser until after the water has been filtered, the reason for this is simple. UV radiation will penetrate clean water far more efficiently than dirty water. So leaving that until last makes very good sense.
Complete filters with a UV steriliser and pump are readily available ready made and simply need plugging in. If you buy a ready made filter make sure that you get the right size of filter with the correct flow rate and with the right UV or it simply won't work.
The simple DIY filter above is made from a domestic cold water storage tank, water flows from right to left as it is viewed. There is an isolation valve to stop the water flow when the filter needs cleaning.
When the water enters the filter it passes through to rows of filter brushes which collect large particles of debris, then it passes through a box made from Japanese matting and which contains a mixture of shells and bio media. The water is then pumped back to the pond via a UV steriliser (just about visible behind the filter box).
One final word about filters and any other pumps in use. Turn them off in winter, if you leave a pump running the water won't form layers and the fish will be placed at risk. Moving the water around may help to prevent it from freezing giving the impression that it is warmer but this really isn't the case for the fish.
Preparing for winter when the fish will be at their most vulnerable. Sometime in late autumn once most of the leaves have fallen is a great time to clean the bottom of the pond by removing as much dead organic material as possible so that it won't pollute the water later on.
Stop all feeding, the fish should be at their peak having fed all summer and they'll slow down for winter and they won't be as interested in food. Also if there is a sudden chill which causes the water temp to drop the fishes digestion will slow down and having a full stomach at such times could be a health risk.
If ice forms on the surface don't break it, it will only re-freeze even thicker than before and don't pour hot water on to it. The best solution is to use a special pond heater, they are like ordinary aquarium heaters and around 100 watts so they won't break the bank. They come fitted with a float which holds them near to the surface and they keep a hole of about 15cm (6 inches) clear of ice. That is all that is required to allow a gas exchange and to keep the water and fish safe.
Goldfish: Some varieties of goldfish will be fine living year round in a pond and others won't.
Ordinary goldfish, shubunkins, commet tailed goldfish, fantails, fantail pearl scaled goldfish will all be fine. The twin tailed, round bodied long finned fancy varieties will probably fall victim to a predator, get bullied or end up with fin rot either way they won't thrive and won't last a year.
Ryukin, orandas, lionheads, ranchu, celestials, bubble eyes, moors and so on really need to be kept in an aquarium at around 18 - 22ºC (65 - 72ºF) even in summer they will quickly fall victim to predators.
Stocking: This can be a problem area for small ponds. Technically you could stock at the same rate as a cold water aquarium but if you did the pond would look so busy and crowded that it wouldn't look right. For purely aesthetic reasons about 1 inch of fish per 10gall of water looks about right, but remember to allow for growth.