Fish: Pond fish
There is a wide range of fish that can survive outdoors through frost and snow, but remember that you want to be able to see the fish, and dark, mottled, or black fish are less visible. You can go the hassle-free route and just pick the healthiest-looking goldfish or golden orfe you can find (free from white spots, fungus or damaged scales) or go the whole hog and read up on Koi carp, though these fish can be very expensive.
Setting up the pond
Generally it is a good idea to have a fairly deep pond, so that your fish can escape more easily from predators, like cats and herons, and are less vulnerable if the surface ices over. The fish will need to be able to hide away under plants (water lilies provide surface shelter, as well as looking pretty) and need water weed to browse on and for oxygen. You can build little caves for your fish, though make sure that the caves are stable, and the fish are not likely to damage themselves by dislodging rocks. Small fountains and other moving water features are useful for helping to aerate the water in summer, though they are not necessary if your pond is large and partly shaded. It is important not to overstock your pond. You need at least 30 sq cm per 2.5 cm of fish, from mouth to tail tip. Koi carp, which can grow up a to metre long, need ponds at least 1.5 metres deep.
Fish being transferred from aquaria are best transferred in summer, when the water is warmer, so it is less of a shock for them. Give new ponds a month or so to stabilise before you put the fish in. You can get fish used to the pond water before putting them into the pond by mixing it with the water they arrive in, in a bucket or other container, placed in a shaded spot, and leaving the fish for half an hour or so. Some people advise floating the bags fish are bought in for an hour on the pond surface, to equalise the temperature. This may not be a good idea if it means keeping a fish in the bag in a small amount of water on the sunlit surface of a pool. The fish may not have enough water for its needs, and will probably only start to relax once it can dart away and hide somewhere.
Keep the pond free of falling leaves and other debris, which can lead to methane-producing silt if left unchecked for long periods. Some types of weed also have to be kept in check so that they don't clog up the pond, leaving the fish little space for swimming. Canadian pond weed can be fast-growing, for example.
Feeding
There is a wide range of fish food available, but use it sparingly. There is more risk of overfeeding the fish than letting them starve, especially for smaller goldfish in a pond. If the fish move away and there is still uneaten food, cut down what you give them.
Health
Pond fish are less likely to suffer from infectious diseases like whitespot if they are given enough space. You can try treating infected fish with various preparations. It is also a good idea to isolate infected fish, and to put new fish in a quarantine tank or pond for a fortnight or so, before letting them join existing fish. Wash all new weed thoroughly to help prevent infection.
Leeches, dragonfly larvae, and frogs can cause problems for pond fish. Check weed for leeches before putting it in the pond, because it takes a long time to clean a pond once they establish their presence. Dragonfly larvae are nasty, vicious, carnivorous creatures without the grace of the adult, and should be removed if spotted. Frogs can get carried away and try to mate with goldfish, so are best kept separately. You can set up a separate pond for frogs, or try to find them another home.
Cats and herons are a bane for pond fish lovers, and you have no option but to cover your pond with a strong mesh if these are a problem. Locating the pond near your house will help to deter herons, though it does not deter cats. Large fish in relatively shallow ponds have 'dinner' written on them in neon lights, so give your fish plenty of depth to escape into, and hidey holes, like clay plant pots on their side, or other artificial caves. You may not always be able to see them as easily, but they'll be around for longer for you to enjoy them!
See also:
News and Research: Fish and Marine Invertebrates, General
News and Research: Fish and Marine Invertebrates, Health, disease and physiology
Reviews of books on fish:
Fish on stamps: http://www.filatelicbird.co.uk/fish.html
