Goldfish are tolerant of most water conditions but a large aquarium is required if you are to keep them well. The old-fashioned goldfish bowl is not suitable: it has limited surface area for the uptake of oxygen into the water, no room for a filtration system, and insufficient swimming space. An aquarium is far better to keep a goldfish happy, and there are companies which produce a packafe which contains everyting including the tank and filtration system - you just add fish, plants, and water. Goldfish are noted for being messy, greedy feeders that produce a lot of waste. Needless to say they require a very efficient filtration system to cope with this. There is a good range of goldfish foods available but don't forget to nclude some vegetable matter as well as live or frozen foods in their diet.
Do not try and overcrowd your fish as this places a great deal of stress on them. If conditions become too warm and the dissolved oxygen level falls, the fish will gasp at the surface, their fins will become clamped to their bodies, and signs of fin congestion (bloodshot bases to the finnage) will appear. A partial water change to lower the temperature, and a check on the filtration system will usually counteract the problem, which could however, also be indicative that your fish are overcrowded.
Breeding Goldfish is relatively easy. In the garden pond it should occur naturally and some of the fry may survuve to maturity. To breed under controlled conditions, condition your chosen pair well on live foods. When ready to spawn the male will develop tubercles on his head, operculum, and pectoral fins; the female will be noticeably rounder.
Use a 100 x 30 x 30 cm (36 x 12 x 12 in) tank with spawning mops suspended in it so that the pair can swim through them, depositing and fertilizing the effs as they do so. Once spawning is complete the pair can be removed and the eggs left to hatch. Feed the fry copious amounts of small live foods. Allow plenty of space for growing on, and cull the numbers rather than crowding if only limited space is available.