What conditions does a goldfish need?
Goldfish (Carassius auratus) can live for many years and reach a substantial adult size. Their system must be designed around adult size, variety, activity and group size rather than the size of the juvenile at purchase. A bowl cannot provide normal swimming space, effective filtration or stable water.
Choose an aquarium for the adult fish
Use a long rectangular aquarium with a large air-water interface. Volume recommendations differ between organisations and cannot replace usable dimensions. As a welfare reference, RSPCA Australia recommends at least 50 litres and a tank four adult body lengths long, two body lengths wide and three body lengths high; increase space with fish numbers and for common varieties, which are larger and more active.
An adult common goldfish may ultimately be better suited to a properly designed outdoor pond. Never transfer one without checking depth, oxygenation, temperature variation, predator protection and rules against release into the natural environment.
Provide mature filtration and stable water
Install mechanical and biological filtration sized for the waste load before adding fish, and allow the biofilter to establish. Regularly test total ammonia nitrogen, nitrite, nitrate, pH and temperature. Rising ammonia or nitrite requires immediate action on feeding, stocking and water changes; simply adding a product does not solve the cause.
Make partial water changes with prepared water of similar parameters, remove settled waste and do not wash all biological media in chlorinated tap water. Maintain effective oxygenation, a stable temperature suitable for the variety and lighting with a true dark period.
Support natural behaviour
Goldfish are social. When space and filtration permit, keep at least two compatible individuals rather than one fish alone. Provide clear swimming space, visual cover, non-sharp plants or structures, and substrate that cannot be swallowed or lodged in the mouth. Avoid varieties or tank mates whose speed, body shape or temperature needs are incompatible.
Follow the guidance on feeding goldfish in an aquarium: offer a complete goldfish diet in small portions and remove uneaten food. Before purchase, make sure the system can be enlarged if required and that care can be maintained throughout the fishes’ lives. For purchases from a professional seller in France, the guide to pet-store health plans explains the establishment’s monitoring procedures.