Goldfish (Carassius auratus) have evolved from the common crucian carp (Carassius gibelio) through extensive domestication and selective breeding processes.

The common crucian carp is native to Asia, and over the course of centuries, breeding and selection efforts have led to the development of various goldfish species with distinct colors and shapes.

The evolution of goldfish involved the deliberate selection of specific appearance traits, including color, body size, and scale shape, as well as personality and growth traits that are well-suited for captive environments.

Through human intervention and dedicated efforts, goldfish have emerged as one of the most popular ornamental fish worldwide.

In terms of biological classification, goldfish belong to the Animalia kingdom, Chordate phylum, Fish class, Cyprinidae family, Cyprininae subfamily, and Carassius genus.

There are more than 300 goldfish species worldwide, which can be broadly categorized into grass species and Chinese species based on their morphology. Some of the notable strains include the dragon species and egg species.

Despite evolving from crucian carp, goldfish exhibit significant changes in their appearance compared to their ancestors.

In terms of body color, goldfish can display silver gray, red, yellow, white, black, purple, blue, transparent, and piebald patterns, among others.

In terms of body shape, goldfish can have lion heads, tall heads, blister bodies, dragon eyes, pompoms, pearl scales, butterfly tails, and tiger heads.

Additionally, goldfish can have different types of caudal fins, including single tail, double tail, triple tail, and vertical tail, as well as specific tail shapes like fantail and butterfly tail.

Various goldfish species include:

1. Grass Goldfish: Grass goldfish represents the original form of goldfish. It features a spindle-shaped body, vibrant colors, a normal dorsal fin, a single caudal fin, and small eyes.

It can be further divided into golden crucian carp, grass gold carp, and red-white flower grass goldfish.

2. Dragon Species of Goldfish: Dragon species of goldfish is a representative variety. It is characterized by a stout body shape, a flat and wide head, well-developed fins, protruding eyeballs, and double and elongated anal and caudal fins.

It is often regarded as the epitome of authentic goldfish.

3. Wen Species Goldfish: Wen species goldfish is one of the oldest goldfish species.

It features a body shape shorter than that of crucian carp, developed fins slightly wider than those of the dragon species, and a head that can be broad or narrow.

The body of the fish resembles the Chinese character "Wen," giving it its name.

4. Egg Goldfish: Egg goldfish is a general term for goldfish without dorsal fins. It lacks a dorsal fin, has no protruding eyeballs, and possesses a short and round body shape resembling a duck egg.

The tail can be either short, referred to as "egg," or long, known as "Danfeng." The other fins are also short.

When it comes to feeding goldfish, several methods are commonly employed:

1. Fish Tank: Goldfish individuals are typically not large, so they can be kept in an ordinary fish tank.

However, if the number of goldfish is substantial, it is recommended to use an aquarium. The material of the fish tank can vary, including glass, ceramic, or plastic.

2. Water Quality: Goldfish possess strong adaptability to water environments, but maintaining good water quality is essential for their well-being. The optimal water temperature for their survival ranges from 18 to 26°C, with a critical range of 0 to 39°C. It is advisable to expose tap water to sunlight or let it stand for 2-3 days before using it for goldfish.

3. Feeding: Goldfish are omnivorous, consuming both plant and animal-based feeds. Plant-based feeds may include rice, wheat grains, and vegetable leaves, while animal-based feeds can consist of earthworms, red worms, and water fleas.

It is recommended to purchase goldfish compound feed from flower and bird markets or online sources and feed them twice a day, once in the morning and once in the afternoon.

4. Water Changes: Regular water changes are necessary to maintain good water quality, as poor water conditions can lead to diseases.

In summer, water changes can be performed every 3 to 4 days, while in spring and autumn, once a week is sufficient.

In winter, water changes can be done every two weeks, with each change involving one-third of the water volume.