Comet Goldfish – A Complete Care Guide

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Goldfish make popular pets due to their low price and easy care requirements. However, some goldfish owners may not be aware of the specific needs of comet goldfish. This guide provides a complete overview of comet goldfish care, including how to select a healthy fish, what to feed them, and how to create a suitable environment. By following these tips, you can help your comet goldfish thrive and enjoy a long life in your home aquarium.

Key Takeaways

  • The comet goldfish is named after its long and forked flowing tail.
  • These fish are most closely related to the common goldfish breed, meaning that they can grow in excess of a foot long and live long lives.
  • Like most goldfish, the comet goldfish creates a lot of waste and does best in a large aquarium or pond setting with strong filtration.

Comet Goldfish Overview

Scientific NameCarassius auratus
Common NamesComet goldfish, Comet-tailed goldfish
Care LevelEasy
Lifespan15+ years
Tank LevelAll levels, mainly mid-level
Minimum Tank Size55 gallons
Temperature Range60ºF to 72ºF
Water Hardness2 to 12 KH
pH Range6.5 to 7.5
Filtration/Water FlowModerate
Water TypeFreshwater
Difficulty to BreedModerate
CompatibilityCompatible with koi and other single-tail goldfish
OK, for Planted Tanks?With caution

What Is A Comet Goldfish?

Most people have owned a goldfish at one point or another in their lives. But did you know that there are many different types of goldfish all with their own unique traits and characteristics?

The comet goldfish is a common breed of goldfish, scientifically known as Carassius auratus. These fish are very similar to the main breed of goldfish, the common goldfish. While these two fish are the same species, there are a few differences between them and other goldfish. As we’ll see, the main difference lies in the shape of their tail fin.

Are They Good Pets?

We might be biased, but we think that all fish make good pets. They’re easy to keep, relatively inexpensive, low on time requirements, and make any area of the home come to life.

Comet goldfish care is relatively straightforward, even for inexperienced keepers. As long as time and research are given to making preparations before impulsively buying the fish, then your comet goldfish should be with you as a pet for years to come.

Why Are They So Cheap?

If comet goldfish make good pets, then why are they so cheap? These fish are often given away as prizes or as supplementary food for larger fish. They are often sold for under a dollar and are found in nearly every pet store that carries fish. There are a few reasons why these fish are so cheap.

The first reason is that comet goldfish are bred on a mass scale as primarily feeder fish. These fish are easy to breed and easy to keep, making them an ideal breed to retail as feeder fish. The problem is that they are often kept in poor conditions which leads them to be very sickly, subsequently spreading disease and illness to each other and to other fish.

The other reason why comet goldfish are so cheap is that they’re not very desirable fish and take up a lot of space. We’re sad to say that there are many more beautiful fish other than the comet goldfish, including fancy goldfish. These fancier counterparts are more ornate and take up less space. This, in addition to the hundreds of other more colorful tropical fish available, leaves many hobbyists choosing something other than a cold water fish.

Origin and Habitat

Goldfish have been domesticated for hundreds of years for their colors and symbolism. These fish were and still are a sign of wealth and prosperity, though many hobbyists adore them for their extreme hardiness and variety of breeds.

The first goldfish were domesticated from crucian carp. These carp originated in China from cold and shallow lakes and ponds. Crucian carp typically feature dusky brown and yellow coloration. Over time, the undertones of yellow and orange were selectively bred until arriving at the intense coloration of the common goldfish today.

However, the breeding didn’t stop there. There are estimated to be well over 100 breeds of goldfish, with the comet being one of the most closely related to the common goldfish.

Comet goldfish and common goldfish do not naturally exist in the wild. In fact, no breed of goldfish exists in the wild. Sadly, many people are unaware of goldfish’s true needs and release them into nearby streams and rivers. Many of these fish don’t survive, but some do. This makes them an invasive species that could potentially impact ecosystems on a large scale1


The comet goldfish is one of the more basic-looking breeds of goldfish. There are a few differences between this breed and the common breed. The biggest difference lies in their fins.

What Does A Comet Goldfish Look Like

Comet goldfish have long and flowing fins that resemble the tail of an astronomical comet. This is in comparison to the common goldfish’s short and triangular fins that follow close behind. The biggest difference between these two fins is that comets have a deeply forked tail fin while common’s have a slight indent in the middle.

For the most part, common goldfish only come in typical goldfish-orange coloration. Comets offer more variety with orange, yellow, red, and white color patterns. 

How Big Do They Get?

One of the most important factors to consider when choosing a breed of goldfish is its potential size. Common goldfish can easily surpass a foot in length, though most stay about 10 inches on average.

Because of their long, flowing fins, comet goldfish size is typically going to be slightly larger, reaching mature lengths of 12 inches. It should also be said that while these fish grow to be long, they can also grow to be pretty chunky too!

Do They Stop Growing?

It’s long been said that fish will only grow to be the size of the tank that they’re given.

Or maybe you’ve heard that fish will have externally stunted growth in small tanks, but that their organs keep growing on the inside.

While this area of fish biology has been little studied, it is true that both these statements are false. The majority of fish, including goldfish, are indeterminate growers that continue to grow with age as long as environmental conditions allow. Though it might seem like your fish has comfortably grown into its tank, it’s actually being stunted due to other factors, including limited space, poor water quality, and in diet. However, its organs will grow in relationship to its body.

How Long Does It Take For Them Reach Full Size?

This is a very common question as many aquarium keepers plan on growing out their goldfish until it’s ready for a bigger aquarium or pond. It’s important to know that goldfish grow rapidly and don’t stop.

Comet goldfish can grow to their mature size in a matter of a year or two. A small fish can quickly become full-grown before you know it. Though a grow-out tank is ideal for an outdoor pond system to prevent predators from eating juveniles, this method is never recommended for a tank-to-tank transfer.

How Long Do They Live? 

Goldfish have extremely long lives and can live for a long time in less-than-perfect conditions. Comet goldfish can live about 15 years with many growing older than this. These fish are a long-term commitment and need to be given plenty of thought before being purchased.

Color Variations

Not all goldfish are gold. In fact, the comet goldfish comes in many different colors besides the traditional copper tone of the common breed. These fish can be orange or yellow and have red and white color patterns. Some of these color combinations have been specifically bred with design in mind.

These designer comet goldfish include:

Sarasa comet goldfish. This variety has a white body with multiple solid bright red spots. This red coloration is typically on the dorsal portions of the fish but can extend to the underbelly as well.

Tancho single-tail comet goldfish. The Tancho coloration is simple yet effective. These fish have a bright red cap on their head while the rest of the body stays iridescent white. This cap may be irregular in shape, sometimes splitting in half.

Care Requirements

Goldfish are an oxymoron. They are an easy fish species to keep, but their care requirements can be surprisingly demanding. While comet goldfish are very hardy fish that can survive less-than-perfect water conditions, they require a lot of upkeep and maintenance to keep them happy and healthy.

Aquarium Setup

Goldfish setups are simple by design. These fish do best with tons of open swimming space, little to no decorations, and strong filtration. No matter what though, these fish cannot live in a goldfish bowl!

Comet goldfish are inexpensive fish that are usually won at carnivals and fairs. They are sometimes even sold as feeder fish for other larger predatory species. Because of their inexpensive price and oftentimes small holding tanks, unknowing goldfish owners think that they don’t need an elaborate setup. While this is true, there are some conditions that need to be met.

A good comet goldfish tank will be a large tank with an appropriate substrate, good water flow and aeration, correct water temperatures, and appropriate tank mates. Comet fish can also be kept in outdoor ponds as they hibernate over the cold months.

Some goldfish enthusiasts keep live plants with their comets. Plants add many benefits to the home aquarium and are essential for keeping a natural environment in pond settings. However, goldfish love to eat and uproot plants, which can add additional waste to the system. Hardy and fast-growing species, like Anacharis and hornwort (Ceratophyllum demersum), can be successfully kept in a goldfish aquarium.

Tank Size

Sadly, many goldfish are kept in improper tank sizes. These are large and active freshwater fish that create a lot of waste. They need space to move around and water volume to help keep ammonia and nitrite levels down.

The minimum tank size recommended for one comet goldfish is a 55 gallon tank. This might seem like a lot, especially when the fish is only a few inches big. However, comet goldfish can grow to over 10 inches in the first couple of years of their life. Too many hobbyists purchase goldfish with the intention of getting a larger aquarium or building a pond. Many times, these plans fall through and the fish is left in too small of a tank (video source).

In order to keep another comet goldfish, a 75 gallon tank is needed. In general, every additional fish requires another 40 gallons of water. This is why these fish tend to do best in a large pond setting, though it is possible to keep multiple comet goldfish long-term in the home aquarium.

Water Parameters

It’s important to keep in mind that goldfish aren’t tropical fish like many of the other species available in the aquarium hobby. They originate from cold waters and need to be kept in cooler conditions in the home aquarium setting. For some hobbyists, this means buying an aquarium chiller to help keep the water temperature down.

The ideal water temperature range for comet goldfish is between 60ºF and 72ºF. Some hobbyists keep their comet goldfish tank above or below these values, but overly hot temperatures can lead to stress while cooler temperatures can lead to hibernation. Ambient room temperature is often enough to keep a goldfish aquarium heated, but extra stability can be achieved through a low-range heater.

On top of colder water temperatures, comet goldfish care relies heavily on maintaining ammonia and nitrite levels. These fish create a ton of waste through their diet and fast metabolism. Most oversized filtration can keep up with processing fish waste, but regular tank maintenance and weekly or daily water changes are also needed; most hobbyists perform upwards of 25% water changes at any given time.


Comet goldfish do not require special lighting. Because they are not commonly kept with live plants, there is no need for high-tech equipment. Instead, comet goldfish can live under LED or fluorescent settings.

There is a chance that your goldfish will change colors based on the intensity of the lighting; a darker light will cause your fish to become darker and vice versa.

Filtration and Aeration

One of the most important aspects of keeping comet goldfish is picking the right filtration. These are big fish with fast metabolisms that create a lot of waste. It is generally recommended to use filtration that is rated for at least 4x the size of the aquarium, with bigger always being better.

Since hang-on-the-back filtration can become too big for the side of the tank, many goldfish keepers use a canister filter. Canister filters are also advantageous as the return nozzles can be pointed downwards to help pick up and remove waste from the bottom of the tank. Multiple hang on the back filters or canister filters can be used for adequate filtration and circulation.

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To help supplement water flow, powerheads and air stones may also be added. Goldfish have adapted to waters with low dissolved oxygen levels, so air stones usually aren’t necessary for increasing oxygen. Instead, they can be efficient at keeping fish waste from resting at the bottom of the aquarium.


Goldfish do best without any substrate in a bare-bottom aquarium setup. In fact, having a substrate can create more work for the owner.

All goldfish, including comet fish, love to dig in and around gravel and sand substrate. They uproot plants, move decorations, and kick up detritus that gets stuck on the bottom. This can become messy over time and lead to problems with water quality. In addition, a bare bottom makes aquarium vacuuming and waste removal much easier, which is essential for keeping a goldfish aquarium clean.

That being said, goldfish have successfully been kept on both gravel and sand substrates. Extra care and maintenance will be needed to keep waste from entering the water column.

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Tank Mates

Once you choose to set up a goldfish tank, there is little you can do to have any other fish besides goldfish. This is mainly because of differences in preferred water temperature, but also because of differing temperaments and behaviors as well as bioload. Goldfish need to be kept with other large and active cold water species, and not many fish meet their criteria or behave as good tank mates.

The best tank mates for comet goldfish are other single-tail breeds, namely other comets and common goldfish, in addition to koi fish. Unfortunately, these pond fish must be kept with like-breeds and cannot be mixed with fancy varieties. Fancy goldfish are too slow and delicate to compete with much more active comets.

Some hobbyists have had success keeping zebra danios (Danio rerio) and dojo loaches (Misgurnus anguillicaudatus) with their comets, but we do not recommend this setup for under 100 gallons.


Goldfish are some of the least picky fish when it comes to feeding them; so much so that they might try to eat your finger!

Comet goldfish are omnivores, which means that they need meat- and plant-based foods. They largely accept most aquarium foods, including live, frozen, and freeze-dried brine shrimp, earthworms, and bloodworms. They will also gladly munch on blanched vegetables, including lettuce, cucumber, and zucchini. Some hobbyists also cultivate easy-to-grow live plants, like Anacharis, to feed their goldfish.

To help keep costs low, comet goldfish can be given high-quality goldfish pellets or flakes.

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Comet goldfish will eat as much food as you give them. This means that food should be given in moderation and any leftovers that happen to evade your fish should be removed to keep waste levels low.


Though easy fish to keep, breeding comet goldfish can be difficult and can usually only be achieved in a large pond setting. There isn’t a huge market for regular comet goldfish in the aquarium trade, so keep in mind that giving fry away can also be challenging.

Because breeding goldfish in an aquarium setting requires very large systems, we will only focus on spawning comet fish in a pond setting. Spawning naturally occurs during late spring/early summer when the water temperature starts to rise. This can be replicated in the home aquarium by using a heater.

First, establish a male and female pair. Females are rounder and more robust than streamlined males. Female comets may also develop protruding anal vents during spawning periods while males may develop white tubercles on the gill covers. When ready to mate, males will chase the female in hopes that she will drop her eggs to be fertilized. Because of this aggressive courtship, at least 2 to 3 females should be kept per every male.

Goldfish Fins

When ready, the female will lay her eggs. Usually, this is near vegetation, a spawning mop, or another safe structure. The male will fertilize them and they will hatch after a few days. During this time, the parents and other goldfish are likely to eat the eggs. For better success, remove the fertilized eggs and move them to a separate system.

When the eggs hatch, the fry will stay toward the substrate as they feed off their egg yolk. As the egg yolk runs out, they will become free-swimming and start to search for food. At this point, they may be offered baby brine shrimp, crushed-up pellets and flakes, and other microscopic foods.

Goldfish fry grow fast, so be prepared to transfer them to their final home within several weeks!

Common Health Problems

Like all aquarium fish, comet goldfish are susceptible to common fish diseases like ich, velvet, and fin rot. Because comets have elongated fins, they can easily develop fin rot if water quality is poor.

There are a few other problems that are unique to goldfish, though.

Two of the main health problems with goldfish are ammonia poisoning and nitrite poisoning. Both of these conditions are a result of poor water quality and are easily avoided. However, poor water quality can also lead to swim bladder disease and dropsy, which can be nearly impossible to treat.

A healthy goldfish always starts with good water quality. Make sure to quarantine new additions for at least 4 weeks and check your tank daily for any changes in appearance or behavior.

Final Thoughts

Comet goldfish aren’t the showiest of all goldfish varieties, but they’re more exciting than the common goldfish. That being said, these are still huge fish that need a lot of space and good tank maintenance to keep them from developing health issues. Due to their size and bioload, they’re best in a large aquarium system or an outdoor pond.

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