The Goldfish (Carassius auratus) is perhaps the most recognizable and one of the most beloved species of ornamental fish among aquarium enthusiasts. In many ways, the history of fishkeeping began with them back in the 3rd or 4th centuries.
Their incredible endurance, low food requirements, ease of care, and beauty make Goldfish stand out in the world of ornamental fish.
|Obviously, there are lots of articles on the internet dedicated to caring for these fish. What sets this article apart is that I will strive to avoid general statements, opting instead to reference specific data, figures, and experimental results.|
This approach is designed to provide you with a genuine understanding and the practical knowledge needed to excel in caring for Carassius auratus.
Interesting fact: The first appearance of the Goldfish in North America dates back to the 17th century. It is believed that Carassius auratus was the first non-indigenous fish to become established in the United States.
Quick Notes about Goldfish
|Other Names||Chinese goldfish or Asian goldfish|
|Scientific Name||Carassius auratus|
|Tank size (minimum)||20 gallons (~80 liters)|
|Size (average)||6 – 8 inches (15 – 20 cm)|
|Optimal Temperature||68 – 74°F (20 – 23°C)|
|Optimal PH||7.0 – 8.5|
|Optimal GH||6 – 15|
|Dwellers||top to bottom-dwelling|
|Nitrate||Less than 100|
|Life span||up to 15 years|
Taxonomy Problems of Goldfish
|The main problem is that Goldfish have been subjected to over 1000 years of intensive domestication, selective breeding, and hybridization. As a result, we have great variability in traits.|
Generally, Goldfish can be classified into at least three species:
- Carassius auratus,
- Carassius carassius,
- Carassius cuvieri.
Among them, Carassius auratusis so variable that its varieties are sometimes treated as independent species or subspecies. For example, many authors have recognized two subspecies in its native range:
- Carassius auratus (Goldfish, Chinese goldfish, or Asian goldfish) from Asia, and Carassius auratus gibelio (Prussian carp, Gibele carp, or European goldfish) from Eastern Europe.
European goldfish likely represent a mix of different lineages and hybrids contributing to taxonomic confusion.
Note: There are also C. (a.) burgeri, C. (a.) grandoculis, and C. (a.) langsdorfii.
Distribution of Goldfish
Originally native to East Asia, including areas of China, Korea, Japan, and eastern Russia, these fish are now widespread all over the world through releases from captivity and aquaculture escapees.
Nowadays, due to their impressive ability to survive, Goldfish have been collected from every US state except for Alaska.
For example, these fish were first taken to Australia and New Zealand in the late 1860s and is now widespread and well-established in the countries.
Globally, Goldfish are considered one of the most widely introduced and spread aquaculture fish species around the world. They can be found on every continent except Antarctica.
Habitat of Goldfish
Goldfish usually inhabit non-moving or slow-moving freshwater habitats like ponds, lakes, marshes, and pools with luxuriant vegetation and muddy bottoms.
They are typically found at shallow to moderate depths, rarely exceeding 65 – 100 feet(20 – 30m) even in deep lakes.
Description of Goldfish
Carassius auratus is a large freshwater fish, with some reaching lengths of 12 inches (30 cm) or more in the right conditions. However, most commonly kept Goldfish are around 6 – 8 inches (15 – 20 cm) in length and 3.5 – 10 oz (100 – 300 g) in weight.
- Body Shape. Goldfish have a streamlined, elongated body that is laterally compressed (flattened from side to side). The body shape can vary depending on the specific goldfish variety, many varieties exhibit more extreme body shapes.
- Color. Wild coloration ranges from olive green to bronze. However, selective breeding has produced varieties with orange, yellow, white, black, red, brown, and various combinations of these colors.
- Eyes. They have large, round, and protruding eyes which are positioned laterally on the head. They can come in various colors as well.
- Mouth. Their mouth is terminal and oblique with no barbels. They have one row of four teeth on each side of their pharyngeal bone, giving them a dental formula of 4/4.
- Spine. Goldfish usually have 29-33 vertebrae in the vertebral column. This high count provides flexibility.
- Scales. Goldfish have scales covering their bodies, which can vary in size and texture depending on the variety. The lateral line contains 28-34 scales running along the side.
- Fins. The dorsal fin is long with 14-19 soft rays and with a slightly serrated third spine. The anal fin has 5-6 rays. The caudal fin is forked. All fins are supported by lepidotrichia – segmented, flexible fin rays.
Interesting fact: When describing Goldfish, Charles Darwin once wrote, “Passing over an almost infinite diversity of color, we meet with the most extraordinary modifications of structure”.
|Through selective breeding over hundreds of years, lots of different varieties of Goldfish (colors and fin shapes) have been produced by man. Nonetheless, if released in the wild, the next generations of Carassius auratus reverse back to normal wild forms and colors.|
Lifespan of Goldfish
The average Goldfish lifespan is 6-7 years in the wild and 10-15 years in captivity.
With excellent care, it has been reported to be as long as 30+ years.
Lifespan varies significantly based on environment and genetics.
Typical Behavior of Goldfish
Goldfish are generally known for their peaceful and calm temperament. They are not aggressive towards other fish and are often kept in community aquariums. However, their behavior can vary slightly depending on the specific goldfish variety.
Goldfish may display aggressive tendencies and nip at each other’s fins. This behavior is usually triggered by factors such as stress, overcrowding, suboptimal water parameters, food competition, and breeding-related behaviors.
Goldfish are pretty active swimmers and can spend a significant amount of time swimming around the tank. They may explore their environment, sift through the substrate, and interact with objects or tank decorations.
Some varieties, like Fancy goldfish with long fins, may swim more slowly and may not be as active as common or Comet goldfish.
Note: Goldfish can also exhibit erratic swimming behavior if they are sick or stressed.
Despite the fact that goldfish are naturally social creatures, they can also be kept individually in an aquarium without causing significant harm to them.
However, when kept in a group, they often engage in shoaling behavior, swimming together in groups. This social aspect of their behavior is more pronounced in some varieties, such as fancy goldfish.
Placement in Tank:
Although Goldfish swim in all layers of the water, they are still known for their bottom-feeding behavior, especially when searching for food.
They will often forage along the substrate, looking for detritus, small invertebrates, or leftover food.
Placement in Tank:
- Social: Yes
- Activity: Moderate
- Placement: Top to bottom
- Peaceful: Yes
- Nippers: Yes
- Jumpers: Yes
In the wild, Goldfish are benthic feeders. It means that they primarily feed on food sources that are found at or near the bottom of the water.
They are also omnivorous, which means they eat both plant and animal matter including insects, insect larvae, fish eggs, fry, crustaceans, zooplankton, algae, benthic vegetation, and detritus.
They forage by sifting through the substrate.
In the aquarium, the Goldfish can be fed a wide variety of meals such as:
- mosquito larvae,
- detritus worms
- grindal worms,
- tubifex worms, etc.
Enhance Goldfish Coloration – Astaxanthin
Color intensity is an important quality criterion for Goldfish value. Unfortunately, they cannot fully synthesize their own carotenoid pigments and require dietary carotenoids.
Astaxanthin and canthaxanthin are common carotenoid supplements that significantly enhance the coloration, particularly their red and orange hues.
Results of the experiments showed that the optimal astaxanthin dosage for Goldfish is around 16.33-16.77 mg/lb (36-37 mg/kg) diet.
- Commercial products. Look for goldfish food that contains astaxanthin as an ingredient. This will improve their vibrant colors.
- Spirulina and Krill. Spirulina and krill are natural sources of astaxanthin. Feeding your goldfish foods that contain these ingredients can help enhance their coloration over time.
How to feed Goldfish
Feeding Goldfish should not pose any problems as they are not picky and will consume almost anything quickly.
This is a diurnal species, so it is better to feed them in the morning.
From a scientific perspective, if you feed the fish every day, you should provide them with food roughly equivalent to 2% of their body weight. If you feed them every other day, which is already undesirable, you should give them up to 5%.
Of course, in most cases, only a few people will bother with such calculations, and therefore the «5-minute rule» remains the best option for the average aquarium enthusiast. The fish should consume all the food within 5 minutes, and if there is anything left, it’s a sign of overfeeding.
Are Goldfish Plants Safe?
No, they are not. I would never recommend keeping them in a nicely planted tank. They are too destructive. Goldfish often uproot and/or nibble on aquatic plants.
If you still intend to keep Goldfish in a planted tank, it is advisable to:
- Choose hardy, fast-growing aquatic plants that can withstand the occasional nibbling or uprooting. Some suitable plant species include Anubias, Java Fern, Cabomba, Amazon Sword, Brazilian Pennywort, Vallisneria, Rotala, etc.
- Keep your fish well-fed.
- Anchor plants well, and use plant weights/barriers. This helps against uprooting and limits Goldfish access.
- Use Fakeaquariumplants.
- Diet Type: Omnivore
- Food Preference: Meat
- Feeding Frequency: Daily
Keeping and Caring for Goldfish
Due to its high tolerance to water pollution and wide-ranging diet, Carassius auratus is regarded as one of the most successful invasive species in aquatic environments.
|At the same time, it is CRUCIAL to consider that many artificially bred forms of Goldfish may not boast the same level of health and resilience as their wild counterparts.
This is because inbreeding, as well as the selection of fish based on certain characteristics, such as shortened body (Goldfish Jikin), excessively protruding eyes (Celestial Eye Goldfish), and so on, have led to potential health issues in some varieties.
As a result, these varieties can be more susceptible to health problems and, consequently, less adaptable to changes in their aquatic environment.
Therefore, it’s essential to maintain these fish under stable and optimal water parameters.
The size of the aquarium and its density depend directly on two main factors:
1. The size of the fish itself.
2. Its activity level, which also relates to its overall need for a specific volume.
Regarding size, experiments related to raising Goldfish fry have shown that their density can be very high. The researchers found that the fry grew the best in the tanks with 300 fish per 10-gallon (40 liters) tank.
However, when it comes to adult fish, for proper aquarium care, it is recommended to have a minimum tank size of 20 gallons (80 liters) for a single Goldfish. If you plan to keep a pair (male and female), you’ll need at least a 40-gallon (160-liter) tank. Keep in mind that these fish can grow quite large!
Bigger is always better as Goldfish are prolific waste producers. Large tanks allow for greater water dilution and swimming space.
Additionally, long tanks are better than tall tanks of the same size.
Note: Goldfish are good jumpers. Especially, slim-bodied goldfish, such as common or comet goldfish, are notorious for jumping out of the water.
Temperature: The general recommendation for a healthy temperature range for goldfish is 68 – 74°F (20 – 23°C).
However, according to the study, after attaining the selected acclimation temperature at the rate of 1 °C per day, fish were held for a minimum of 20 days at constant temperatures of 41°F (5°C), 59°F (15°C), 77°F (25°C) and 95°F (35°C).
In the wild, Goldfish can tolerate high temperatures and survive winters outdoors.
PH: Another experiment showed that Goldfish are extremely tolerant of environmental stress including high levels of turbidity and fluctuations in pH. Laboratory tests have revealed pH tolerance levels between 4.5 and 10.5.
In the aquarium, it will be better to keep pH levels between 7.0 and 8.5.
Hardness: This species does well in soft or hard water (6 – 15 GH).
Salinity: Carassius auratus exhibits good growth and no signs of stress in saline waters up to 6‰ salinity. In fact, low salinity (1‰) exposure reduces the mortality of Goldfish experimentally challenged with bacterial pathogens. Higher salinities (8 and 10‰) produce significant muscle dehydration, significant increases in circulating cortisol, and adverse effects on growth, food intake, and food conversion rate.
Note: Goldfish have been captured in waters with salinity levels as high as 17 ppt.
Dissolved oxygen: Goldfish exhibit a striking capacity to cope with low levels of oxygen and even prolonged periods (ranging from hours to days) of total anoxia. They are able to exploit an anaerobic, or mixed aerobic-anaerobic, metabolism.
According to the study, Carassius auratus has a one-half-lethal time of 45 hours under anoxia at 41°F (5°C) and 22 hours at 68°F (20°C).
Ammonia: Goldfish can tolerate high levels of ammonia. In one of the studies, it was found that its maximum tolerance to ammonia was recorded as 9 mM (96-hour LC50). 9 mM × 17.03 mg/L/mM = 153.27 mg/L
Nitrate: Almost the same can be said about the nitrates. Goldfish in the experimental group were exposed to significantly higher nitrate levels (48.5 mg/L) compared to the control group (8.5 mg/L). All fish survived in both groups.
Nonetheless, even though Goldfish are generally considered tolerant of high nitrate levels, but long-term exposure can still lead to health issues and reduced overall well-being.
Copper: Genus Carassius has a higher tolerance to copper compared to other freshwater species.
Goldfish were subjected to 2 different copper concentrations: a high concentration of 0.84 µM (for 1 day) and a low concentration of 0.34 µM (for 7 days), both of which were below lethal levels.
Good filtration is highly recommended to maintain water clarity and quality.
Goldfish have earned a reputation as aquarium “mess-makers,” and there’s good reason for that.
These fish are active and quite voracious eaters, which means they produce a significant amount of waste in the water. Additionally, they also tend to stir up debris by constantly sifting through the substrate. Therefore,
Although Goldfish can survive even in total anoxia it is still not a good environment for them.
Aeration helps maintain a healthy and oxygen-rich environment for Goldfish, which is crucial for their well-being.
In the natural environment, Goldfish are found in still or slow-moving waters. Therefore, none or very slow water currents are recommended for tanks housing this species.
Adequate water movement helps oxygenate the water, which is crucial for the well-being of goldfish.
Subdued lighting will be the best choice for the Goldfish.
However, if you decide to keep these fish in planted tanks, lighting should be adapted to the needs of plants.
It is best to use smooth, rounded gravel and stones as the substrate because Goldfish love to dig around in it. Choose a size that prevents the fish from swallowing the pebbles.
When decorating the aquarium, keep in mind that some types of Goldfish with modified eyes (like Telescopes, Celestial eye, Bubble eye, etc.) are prone to eye injuries, so decorations should not have sharp edges.
Note: Goldfish are notorious for reducing water clarity by roiling up sediments. Therefore, some aquarists prefer a bare-bottom tank, which means having no substrate at all. This makes cleaning the tank exceptionally easy and prevents any substrate-related issues. The main downside though is that it does not create a natural appearance.
Like all cyprinids, Goldfish are egg-layers. They produce eggs that attach to aquatic vegetation.
There is no parental care for the eggs or fry.
In the study, it was found that:
- A small percentage (7.55%) of one-year-old females (5 inches or 12.20 cm of total length) are capable of spawning,
- the majority of females (85% of the population) reach sexual maturity by their 2nd
- 100% of females reach maturity by the 3rd
According to the study, it is not possible to differentiate between the sexes of Goldfish until they become mature. After that, there are several factors that distinguish males and females.
|General body shape||Thinner, longer, and symmetrical from the top||Fatter, shorter, and asymmetrical from the top|
|Tubercles||Appear on head, Do not show breeding tubercles operculum, pectoral fins, and other fins||Do not show breeding tubercles|
|Abdomen||Smaller, slender, and firm and may have ridge||Large, fatty, no abdominal ridge, and circular in outline|
|Genital opening||Long, concave, and smaller opening||Convex, large and protruding outside|
|151 ray of pectoral fin||Thicker edge and more pointed||Thinner edge and round pectoral fin|
|Lead (main) ray of anal fin||Thinner||Thicker|
|Behavior||Chase the female||Chased and harassed by male|
In the wild, Goldfish usually begin to spawn at the end of May and are accustomed to spawning twice or thrice in the same season.
When males are ready for spawning, they start nudging the females with their heads and fins to induce them to mate.
Despite the fact that Goldfish do not form pairs, females still show a preference for larger males and spend more time with them, according to observations.
They push females toward spawning sites like plants. After that, males release the milt by rubbing their bodies against the female by aligning on either side of the female.
The eggs are fertilized externally.
Goldfish are capable of producing large numbers of eggs during spawning. Observations showed that fecundity varies from 286 to 219104 eggs, averaging about 46000 eggs.
Egg production is correlated with the size and age of the female goldfish – larger and older females tend to be the most fecund.
Not all eggs are fertilized.
The eggs are spherical in form. The diameter of mature eggs usually ranges from 0.74 to 1 mm. They are white in color and translucent.
Ripe eggs are demersal and adhesive in nature in living conditions. The adhesive nature of the egg appears to be attributed to the very thin porous membrane lying on the outermost part of the egg.
The incubation period depends on the temperature. For example, it is around 74 hours at an average water temperature range of 59 – 68°F (15 – 20°C). All the eggs hatch in approximately 90 hours.
- After hatching, the Goldfish fry are transparent and measured 0.051-0.067 inches (1.3- 1.7 mm) of total length with a large oval head, a well-defined yolk sac, and a short tail (“an eyelash with two eyeballs”).
- 2-4 days old fry reaches 0.098-0.114 inches (2.5-2.9 mm) of total length. They show a distinct reduction in the size of the yolk sac.
- 7-8 days old fry are about 0.173-0.189 inches (4.4-4.8 mm).
- 15-18 days old fry often become 0.228-0.256 inches (5.8-6.5 mm) of total length. However, they usually do not resemble the parents in terms of coloration. They are a metallic brown like their wild ancestors.
- 40 days old young fish are about 1 inch (2.5 cm) in body length.
- After 2-3 months the young fish begin to develop coloration similar to that of their parents.
Newly hatched fry can initially eat egg yolk, then cyclops and daphnia for rapid early growth.
Experiments showed that after testing three diets over 3 weeks – Artemia nauplii (D1), Artemia + 50% dry feed (D2), and 100% dry feed (D3).
- Growth rate and survival were highest on D1 and D2 diets. Growth was slowest on 100% dry feed diet D3.
- Liver and hepatocyte structure showed no abnormalities on D1 and D2. Anomalies were seen in fish fed D3.
- The survival rate was 98-98% on D1 and D2 compared to 91% on D3.
- Weight was 0.007 ounces (200 mg) and 0.005 ounces (150 mg) on D1 and D2, but only 0.002 ounces (50 mg) on D3.
How to Initiate Spawning
- Prepare the Tank. Provide spawning mops, nylon nets, or plants for the collection of sticky eggs. Keep water very clean.
- Increase feeding. Feed Goldfish on 5% body weight and the feeding rate was 3 times per day.
- Separation. Ideally, before the breeding period male and female fish should be placed in separate breeding tanks.
- Increase the temperature. Gradually increase water temperature and light exposure to mimic natural conditions and prime fish for spawning.
- Sex ratio. The sex ratio of the spawners should be at least 2:1 for males and females. The highest fecundity was observed when the sex ratio was 1 female to 3 males, followed by the ratios of 1 female to 2 males and 1 female to 1 male.
In 36 hours, the female releases her eggs, and the male releases sperm for fertilization. These fertilized eggs can be gathered on a nylon net and subsequently transferred to a separate tank.
|Goldfish do not exhibit parental care. On the contrary, they are cannibalistic, therefore, brooders must be separated from the spawning tank soon after spawning.|
In mass production, hormones like Ovaprim are often used to encourage spawning. Experiments showed that the fecundity and hatching rate of eggs in Goldfish bred through induced methods were significantly higher compared to those exposed to natural breeding methods. Additionally, the survival rate was observed to be higher in fish bred through induced methods.
GoldFish and Suitable Tankmates
When choosing tankmates for goldfish, it’s essential to consider their size.
Avoid small fish that can fit in a Goldfish’s mouth, as they will be viewed as food.
Keeping goldfish with shrimp is ABSOLUTELY not recommended. Goldfish are omnivores and will readily eat any shrimp (baby or adult) if given the chance.
Dwarf shrimp may be able to avoid Goldfish initially in a heavily planted tank but likely will not survive long term. Shrimp will be stressed all the time and their survival and breeding compromised.
Certainly, someone may tell you that Goldfish don’t eat freshwater snails, and their aquarium is proof of that.
In reality, it’s not quite that simple, and what works for some may not work for others.
By nature, Goldfish are omnivorous; they can eat just about anything (snails as well), even when they have plenty of other food. This includes not only small snails but also larger ones like apple snails.
It’s just in their nature. Additionally, they may nibble on the antennae of snails. So, keep this in mind.
After over a thousand years of domestication, the Goldfish remains one of the most popular and recognizable aquarium fish. Through selective breeding, Goldfish exhibit stunning diversity in color and form.
Generally, these are hardy and large fish that can be easily recommended for beginners.
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