Cold Water Fish

Goldfish, Carassius Auratus

  • Scientific name: Carassius auratus, Cyprinus auratus
  • Common Name: Redfish, Goldfish, Goldfish, Cometfish
  • Aquarium size: 150 to 200 liters
  • Temperament: Calm
  • Temperature:  Between 16º and 20ºC
  • pH: Between 7
  • Diet: Omnivores
  • Length: Up to 35 cms



Goldfish, red fish, goldfish or Carassius auratus , may be one of the best known and most captive-bred cold-water fish in the world.
Over the hundreds of years that this fish has been bred in captivity, hundreds of spontaneous and forced mutations have been generated , leading to a number of strange and surprising morphological variations.
Goldfish are «unfortunately» associated with crystal ball shaped fish tanks , which of course are not the right container for these fish.

A fish tank is not suitable for red fish, I would even say that for any fish, since it is impossible to condition the container with aquatic plants, no filtration system, no aeration, which leads to the fish being stressed and dying prematurely.
If you have made the decision to buy a Goldfish for your aquarium , you should know that you have made an excellent choice, not only because it is a very popular fish, it is also very resistant, ideal for beginners in fish farming.


Talking about a «morphology» that defines the red fish is impossible. Under the denomination, carassius auratus, red fish or goldfish, there is a huge variety of fish, each with its own morphology.
Some have a dorsal fin, others do not, some have an ovoid body, others on the contrary have an elongated body.
Most have an orange color, although others are more reddish, white, black, blue or metallic green….
The same thing happens with the size of the red fish, some can measure up to 35 centimeters in length or more, depending on the variety and the conditions of the aquarium or pond where we are raising them.
To get an idea of ​​the morphological differences and types of red fish that we can get,These are two of the most original varieties of red fish:

Red lion head fish

The red lion head fish with a red hat is one of the largest varieties, reaching 35 centimeters.
They are ovoid in shape, have a well-developed dorsal fin and a tail fin cut into four lobes, sometimes with stripes.
The name «red hat» is due to the fact that in adulthood (about two years), it presents bumps on the head, which give the sensation of being a beret or hat.

Telescope fish

The telescope fish is representative of how different the fish can be that we find under the name Carassius auratus.
It is a fish native to China. There are references to it between 1848 and 1925.
Well, we can find this fish with a dorsal fin and without a dorsal fin, with slightly bulging or very bulging eyes.
One of the most famous is a black telescope, but it is that within the variety of colors, there are even scales that are totally without pigmentation.
We can be before a telescope fish and think that it is one of the other varieties with hypertrophied eyes that have been obtained.
See: Goldfish Telescope


The Shubunkin is a really interesting variety of the Carassius auratus.
Its main attraction is in its coloration, since the morphological differences are scarce, although it also has them.
It is a fish with a bluish background in most cases, which is then dotted with different black and red spots, even in more original colors such as yellow.
Within the Shubunkin we can find different varieties, as with the Telescope or Lion’s Head.
I invite you to visit its file and learn more about this interesting Goldfish:
See: Shubunkin

Distribution and habitat

The origin of the red fish is in East Asia , being China where they were most popular.
Currently it is possible to find different varieties of carp, all over the world, in any river, stream or swamp. Its adaptability is enormous.
Its original habitat and where it is most comfortable, are the rivers of cold water and slow flow . It is able to withstand the low temperatures outside, without any problem.

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How to care for a Goldfish

Red fish are very strong, so much so that they tend to be very common in outdoor ponds , enduring all kinds of “extreme” situations.
I am sure that, if you have bought them for an aquarium, you will want them to be in the best possible condition, and regardless of how resistant they are, it is best to provide them with adequate care.

Aquarium conditions

Although they are more pond fish than aquarium fish, it is possible to have them in an aquarium without any problem.
They require aquariums of good size, between 40 and 60 liters per specimen , which gives us an aquarium of 80 liters to 120 liters if we want to keep a couple in captivity, between 150 and 200 liters, if we want to have a few specimens. They are fish that grow a lot, before deciding which variety of the multiples that exist for sale, ask what size it will be as an adult and calculate which aquarium you need.
We will look for a warm room and far from any source of heat. The bottom must be of coarse sand, under which we will place a good aeration and filter system.
We can plant plants, as long as they are resistant, since it is possible that the red fish will try to eat them. If we see that we are not successful with natural plants, we can change them to artificial plants.
The ideal water temperature is between 16º and 20ºC, but in an outdoor pond it can withstand variations between 5º and 25ºC.
The pH should be 7, neutral and with a degree of hardness between 5 and 10.

Maintenance of an aquarium for Goldfish

In addition to the temperature, hardness and pH of the water, we must take into account the type of fish we are raising.
They are fish that produce a lot of waste. The filtration system has to be powerful, a little above the manufacturer’s recommendation.
If we have a 150 liter aquarium, the ideal is to have a filter for a 200 liter aquarium .
The water changes must be frequent, at least once a week, removing 15% of the total.
Remember that the new water must be clean of chlorine, and at the same temperature as the tank. We can use water conditioners to remove chlorine and any heavy metals that could affect the fish.

Aquarium cleaning

  • Frequent water changes are a guarantee to avoid the appearance of algae, but they are not an insurmountable barrier … due to the very nature of Goldfish, we can have problems controlling nitrates and phosphates , being useful to apply an antialgae periodically .
  • To clean the bottom, we have a very useful tool: siphons . We can clean the bottom every week, at the same time that we carry out periodic water changes.
  • If we observe that the aquarium walls are dirty, in no case can we use cleaning products (inside).
    There are some sponges that can be used from the outside, without having to put your hand inside the aquarium (not suitable, especially if we have creams or colonies).
  • If we have to extract any decoration for cleaning, we must use gloves, and clean it with warm water without detergents, rinsing very well before putting it back into the aquarium.

What do Goldfish eat?

They are omnivores, feeding on plants, insects and small crustaceans . The typical brine shrimp, shrimp or larvae will be an ideal food, but we should not abuse it, since they are very gluttonous. It is best not to give them too much protein, just a couple of times a week.
On the market there are prepared meals specifically for goldfish, based on feed and flakes, with a higher concentration of carbohydrates and few proteins.
In addition to commercial preparations, we can make our own food, in which we will include boiled vegetables and cut into very small pieces. Some of the allowed foods are corn, beans, rice, peas, cucumber, carrots or zucchini.
Freeze-dried shrimp and bloodworms can be added. The resulting paste must be solidified with neutral gelatin, so that it is floating in the aquarium water and they can eat it without problem.

How many times a day should Goldfish be fed?

The normal thing is to feed them two or three times a day, only the amount of food they can eat without sinking to the bottom.
Remember that the food that is not eaten will go to the bottom, decomposing and altering the quality of the water.
If you go on a trip, you should know that they are very resistant fish, they could go up to two weeks without eating, but it would be unnecessary cruelty.
For travelers there are many alternatives, such as automatic food dispensers or specific preparations for vacations.

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Behavior and compatibility

They are gregarious fish, so it is advisable to have several specimens, as long as the aquarium allows us.
From very peaceful nature, they do not attack any other variety of fish. If we are going to place several types of fish in the aquarium, the only precaution is that they are not too small, because they could serve as food for them.
A good idea could be to mix several types of carassius, although if they reproduce we can give rise to a new variety.

List of fish compatible with Goldfish

They are very calm fish, which can share space with a good number of species:

Compatible fish without restrictions

  • Harlequin fish
  • Snail Apple
  • Female betta fish
  • Gourami Pearl
  • Molly fish
  • Otocinclo
  • Neon Tetra
  • Telescope
  • Tiger Barbel
  • Koi


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