Carassius auratus

Carassius auratus, commonly known as the Goldfish, is a hardy freshwater species that presents a medium level of difficulty, primarily due to its specific housing and maintenance needs. A common mistake is underestimating their adult size and bioload. With a maximum size of 30 cm and a very high waste production, an 80-liter tank should be considered the absolute minimum for a single young fish, with significant upgrades necessary as it grows. For a community of goldfish, much larger volumes are essential. Their high metabolism and oxygen consumption demand robust filtration that can process a large volume of water without creating excessive flow, which they dislike. A canister filter with a spray bar is an excellent choice to ensure high oxygenation and clean water while keeping the current gentle.

Water chemistry should be kept stable. While they tolerate a broad pH range and moderate water hardness, rapid fluctuations can cause stress. They thrive in cooler temperatures, making them unsuitable tank mates for most tropical fish. As omnivores, their diet must be managed carefully. A staple of high-quality sinking pellets is recommended to prevent air ingestion and potential swim bladder issues, a common ailment in round-bodied varieties. This should be supplemented with blanched vegetables like peas and spinach, as well as protein-rich frozen or live foods such as daphnia and brine shrimp. Feed daily, but only what they can consume in a couple of minutes to avoid overfeeding and polluting the tank.

Goldfish are shoaling fish that exhibit social behaviors. However, care must be taken when mixing varieties. Fast-swimming, fusiform types like Comets and Shubunkins will easily outcompete slow-swimming, globiform types like Fantails and Orandas for food. It is best to house varieties with similar body shapes and swimming speeds together. Their environmental interaction includes uprooting and eating most live plants. If live plants are desired, robust species like Anubias or Java Fern tied to driftwood are the best option. Breeding can be induced by simulating seasonal temperature changes but requires a separate setup, as they are egg-scatterers and will consume their own eggs and fry.

Carassius auratus, a domesticated cyprinid originating from East Asia, is a species of significant physiological and morphological diversity. Taxonomically classified within the family Cyprinidae, its wild progenitor is considered Vulnerable (VU) by the IUCN, though its domesticated forms are globally ubiquitous. This species is fundamentally a freshwater organism, though it exhibits tolerance to slight salinity up to 0.5 ppt. Its physiological requirements reflect its high metabolic rate. The notable oxygen consumption necessitates high levels of dissolved oxygen, a factor exacerbated by increasing temperatures within its acceptable range of 18-24°C. The species' high bioload factor (10) is a direct consequence of its size, activity level, and digestive physiology. As a cyprinid, C. auratus lacks a true stomach, leading to a continuous digestive process and consequently, high waste production, primarily in the form of ammonia. This necessitates robust biological filtration in captive environments to manage the nitrogen cycle effectively.

C. auratus demonstrates adaptability to a wide range of water chemistry, tolerating pH levels from 6.0 to 8.0 and water hardness from 90 to 340 ppm. However, homeostatic stability is crucial to prevent osmotic stress. Morphologically, selective breeding has resulted in distinct body plans, principally fusiform and globiform. The ancestral fusiform shape is hydrodynamically efficient for active swimming throughout the water column. In contrast, the derived globiform shape, seen in 'fancy' varieties, compromises swimming ability and is often correlated with health complications, including digestive and swim bladder disorders. As an omnivore, its natural diet is varied. In captivity, a diet mimicking this diversity is crucial for long-term health. Socially, it is a shoaling species, although reproductive events are characterized by broadcast spawning with no subsequent parental care. Breeding is typically triggered by environmental cues, specifically photoperiod and thermal shifts, characteristic of temperate seasonal breeders.