Goldfish

About the species

The goldfish (Carassius auratus) is a domesticated coldwater fish from China that many people underestimate because of small ornamental varieties and the widespread "fish in a bowl" myth – it is in fact a social, long-lived species with surprisingly large space and filtration requirements. Unlike the guppy, neon tetra or angelfish, all tropical species that need a heater, Chinese breeders domesticated and colour-selected the goldfish from a wild crucian-type ancestor (Carassius gibelio/auratus) centuries ago, making it one of the oldest ornamental fish species kept in the hobby at all.the goldfish is a coldwater fish and suffers long-term in heated tropical water.

A coldwater fish, not a tropical one

The goldfish does best at temperatures between roughly 10 and 24°C, with a stable value around 20-24°C being ideal, clearly lower than tropical species such as the angelfish, which needs water up to 30°C. In a suitably heated room a heater is generally not needed; what matters more is that temperature does not swing by more than a few degrees within a single day.

The bowl myth and real space requirements

A goldfish kept in a small bowl does not "stay small" – growth is stunted by stress and poor water quality, which actually shortens its life and harms its health rather than benefiting the fish. For a small group of two fancy (ornamental) varieties, at least around 150 litres is recommended, while single-tailed varieties such as the common or comet goldfish, which grow considerably larger, need substantially more space long-term, or an outdoor pond.

Water and filtration

Because of the large amount of waste it produces, the goldfish is one of the more demanding species to filter for, despite its reputation as a simple beginner fish; recommended pH is between 6.5 and 7.5. Weekly partial water changes of 10-25% and regular testing of ammonia, nitrite and nitrate matter more for this species than for many smaller tropical fish.

Varieties: single-tailed and twin-tailed

Single-tailed varieties, such as the common and comet goldfish, are faster swimmers and can grow beyond 30cm, while twin-tailed ornamental varieties, such as the oranda or telescope eye, are slower and more vulnerable, so mixing the two in one tank is not recommended – the faster varieties often out-compete them for food.

Company and tankmates

The goldfish is social and is recommended to be kept at least in a pair or group of its own kind; suitable tankmates are other coldwater species, such as white cloud mountain minnows or some danio varieties, while tropical species are not suitable neighbours because of the mismatched temperature.

Diet

The goldfish is omnivorous and in the wild feeds on plant material, small invertebrates and detritus; in the aquarium it readily accepts quality goldfish pellets, supplemented with occasional greens such as blanched peas. Overfeeding quickly worsens water quality given the already large amount of waste produced.

An outdoor pond as a long-term option

For single-tailed varieties that grow beyond 30cm, an outdoor pond with sufficient depth is often a better long-term solution than even a very large indoor tank, since it allows more natural behaviour and makes maintaining water quality easier at a larger volume. Before moving fish outdoors, it is worth checking that the pond does not freeze to the bottom in winter, since a goldfish becomes dormant at very low temperatures but cannot survive its surroundings freezing solid.

Lifespan and long-term responsibility

With good care, a goldfish often lives 10 to 15 years, and some varieties considerably longer under suitable conditions, something many beginners who get one as a "first fish" underestimate. Choosing a goldfish is a long-term commitment comparable to many dog or cat care decisions, not a quick, cheap choice for a school fair prize.

Pros & cons