A person responsible for a goldfish has the same duty under the Animal Welfare Act as any other pet owner to ensure the needs of the fish are met.

We would discourage keeping fish in bowls as it can be difficult to properly provide for the animals when kept in this way. Generally, the smaller the container, the quicker the water quality will deteriorate (through evaporation and pollution by waste and uneaten food). You should buy the biggest tank your house and pocket can accommodate. This is because larger volumes of water offer more stable temperatures and water chemistry. The likelihood of compatibility problems between fish will also decrease because the animals can get a reasonable distance away from each other. The general rule is to allow 2.5cm (1 inch) of fish to 4.5 litres (1 gallon) of water, with a minimum of 45 litres (10 gallons). Note that this is based on fully grown fish, ie. ones that will not increase in size. If you are stocking with young fish, it is vital to take into account any increase in size that will take place as the animals grow and base the calculation on their adult size. Otherwise, as they grow, the animals will rapidly become too large for the volume of water, compromising water quality and leading to welfare problems for the fish.

Whether the container in which the goldfish is being kept is adequate depends on the size and number of fish, the size of the fish bowl or tank and whether the environment provides for the needs of the fish. For example, the volume of water should be large enough to provide an environment that is not vulnerable to normal changes in the surrounding air temperature. The container should also produce a large water surface area, which allows efficient oxygenation of the water. The general rule for cold-water fish is that a minimum water surface area of 60cm² is required for each 1cm body length of the fish (excluding its tail), but tanks should ideally be stocked gradually and the water quality monitored to ensure the tank doesn’t become overpopulated. If stocking with young fish, the same consideration of growth and eventual adult size should be taken into account here as above. Larger tanks can be fitted with a pump, filter, lighting, thermometer and a ventilated cover to help maintain the required stable environment. Water plants are also important, as they provide a source of oxygen and a place of safety to reduce any disturbance of the fish.

If a goldfish shows signs of distress (for example, lying still on the bottom of the tank, floating and gasping at the surface, loss of appetite, hiding for unusually long periods of time), expert advice should be sought. We would investigate cases where it is believed a fish's needs are not being met.

http://www.rspca.org.uk/utilities/faq/-/question/ENQWADAWAFishInGoldfishBowls