Freshwater Fish · Goldfish

Comet Goldfish

Carassius auratus

Slim, fast pond goldfish with a long forked tailAdvanced

Lifespan: 10-20+ years
Size: 12-14 in
Social needs: Social; keep 5+ goldfish together
Time: Moderate-high (heavy waste, pond upkeep)
Cost: High
Diet: Omnivore (sinking pellets, blanched greens, less in cold water)

Overview

What is this animal?

The comet is a single-tailed goldfish developed in the United States in the 1880s from the common goldfish, and first seen in the ponds of the US Fish Commission in Washington, DC.
It is slimmer than a common goldfish, with a long, deeply forked single tail that the UK breed standard asks to be about three-quarters as long as the body.

Comets are hardy, active and long-lived: they regularly reach 12 in, can reach 14 in, and can live 10-20 years.
They are sold cheaply as small juveniles, but an adult comet is a pond fish that needs a garden pond, or an aquarium as big as one.

Temperament

Comets are peaceful, social and very active, and they jostle for food at feeding time.
Keep a group of at least five, never one alone.

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They are fast swimmers that outcompete slow, round-bodied fancy goldfish for food, so never mix the two: in a study of four goldfish forms, comets swam as efficiently as commons, while the round-bodied forms used about twice the energy to cover the same distance.
Commons, shubunkins and other single-tails of a similar size are the right company, and tiny fish will be eaten.

A good fit for

Pond keepers with room for a large, deep garden pond
Keepers who want an active, hardy coldwater fish
Single-tail ponds with commons or shubunkins
People committed to 10-20+ years of care

Common mistakes to avoid

Being sold as a small, cheap tank fish
Mixing with slow, round-bodied fancy goldfish
Underestimating 12-14 in adults and the pond they need
Torn tails from sharp decor or fin-nippers

Enclosure

Minimum 680 gallons, for a group of 5

Cannot be kept alone. The minimum is 5, and the space above is for that group.

Comets are coldwater fish that do best at about 50-68°F, need no heater, and tolerate the seasonal swings of an outdoor pond.
Give a group of at least five a garden pond or tank at least 11.5 ft long with at least 3.5 ft of water, about 680 US gallons or more, fully cycled and with strong filtration rated above its volume.
The length is ten times, and the depth three times, the full adult length of the largest fish, here 14 in; comets that stay at 12 in need 10 ft and 35 in.

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A tank that size is a custom build, which is why a pond is the usual home for comets.

Keep ammonia and nitrite at zero and nitrate low with regular partial water changes, in slightly hard, alkaline water of about pH 7.5-8.5.
Use smooth stones and plants rather than sharp decor, net the pond against herons, and never keep a comet in a bowl or a small tank.

Check your enclosure

Supplies

Setup

Pond liner and underlay
Pond pump
Pond filter with UV clarifier
Edging, plants and predator netting
Liquid test kit
Dechlorinator and starter supplies

Ongoing

Sinking goldfish pellets or gel food
Frozen peas (to deshell and blanch)
Courgette and spinach for blanching
Veg clip
Frozen bloodworm or daphnia

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Small cup for pre-soaking pellets
Fine net for removing leftovers

Safe substrate: Fine, smooth sand · Smooth, rounded fine gravel (not for loaches or sand-sifting cichlids; sand is better for corydoras) · Bare bottom (quarantine and grow-out only)

Work out the cost

Care routine

Daily

Check temperature and equipment
Feed a sinking, pre-soaked food
Head-count and look everyone over

Every few days

Top up evaporationevery 3 days

Weekly

Large water change
Test the water
Wipe the front glass only

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Every few weeks

Rinse the filter mediaevery 2 weeks
Trim and thin the plantsevery 2 weeks

Monthly

Check supplies and reagent dates
Replace or recharge chemical media, if you use any (skip it in a blackwater tank)

Health review: Quarterly water and buoyancy review, every 3 months

Tools

Is your enclosure big enough?Check your measurements against the recommended size What will it cost?Setup, monthly and over its whole life, itemised Is this ammonia reading dangerous?Works out the toxic part from your pH and temperature

Care in depth

Diet

Omnivore (sinking pellets, blanched greens, less in cold water)

Comets are omnivores that always act hungry, so they are easy to overfeed.
Feed a quality sinking goldfish pellet as the staple, with blanched vegetables such as peas, courgette and spinach, and occasional frozen bloodworm or daphnia.

Feed small amounts once or twice a day, only what they finish in a couple of minutes.
In a pond, cut feeding right back once the water cools below about 46°F, because goldfish digest food poorly in the cold.

Health

Signs of good health, and the common problems

Comets are hardy, and most of their problems trace back to the water: fin rot, white spot (ich) and other infections follow an undersized system, weak filtration or overfeeding.
Watch for clamped or ragged fins, gasping at the surface, flashing against decor, and white spots on the body and fins.

Keep sharp decor and fin-nipping fish away from that long tail.
Quarantine new fish for 2-4 weeks before they join a pond or tank, because a disease brought in puts every fish there at risk.

Is this urgent?

6 warning signs and how fast to act, 3 of them right away

In a fish tank the patient is usually the water. Test before you treat: dosing a healthy tank for a disease it does not have makes things worse.

Go now

Any detectable ammonia or nitrite in a stocked tankLarge water change now with dechlorinated, temperature-matched water. Stop feeding. Test again in 12 hours. Both are directly toxic. How toxic the ammonia reading is depends on pH and temperature, so the same number can be harmless or lethal.

Now

Fish gasping at the surface with fast gill movement, or crowding the filter outflowIncrease surface movement now (gently in a betta or gourami tank, without creating a strong current), then test the water. Either the oxygen has crashed or the gills are being chemically burned. Both are hours-scale problems. Bettas, gouramis and corydoras rise to gulp air as part of normal breathing; the warning is constant gulping, fast gill movement, or the whole group hanging at the top.

Now

Sudden deaths, more than one fish in a dayTest ammonia, nitrite, nitrate and temperature before doing anything else. Do not add medication yet. Multiple simultaneous deaths usually point at the water or the equipment rather than a pathogen, so rule those out first.

Now

Today

Flashing: repeatedly scraping against decor or substrateTest the water, then look closely for parasites. Irritation is either chemical or parasitic; the test tells you which before you medicate.

Today

White salt-grain spots over the body and finsBegin the appropriate treatment today and check the temperature. The parasite's life cycle is temperature-driven, and it multiplies with every cycle you wait.

Today

Soon

Clamped fins, hiding, or refusing food for more than two daysTest the water and review recent changes. Usually the earliest visible sign of a parameter drifting.

A few days

Never add a new fish to a tank that is not fully cycled, and never treat blind. A test kit is cheaper than the fish.

This is general guidance for a healthy adult, not a diagnosis. An exotics vet who knows your animal always overrides it, and the time to find one is before you need one.

Bedding and substrate

3 safe, 4 with a caveat, 2 to avoid

Substrate in an aquarium is a filter as much as a floor. Most of the biology lives in and on it.

Safe

Fine, smooth sandCorrect for corydoras, kuhli loaches, eartheaters such as rams and acaras, and anything else that sifts or burrows. Barbels wear away on sharp gravel.

safe

Smooth, rounded fine gravel (not for loaches or sand-sifting cichlids; sand is better for corydoras)Rinse thoroughly before use. Grain size should be too large to swallow and too small to trap food. Kuhli loaches need sand to burrow into and rams and acaras sift mouthfuls of it; bronze and panda corydoras manage on smooth fine gravel, but sterbai corydoras need sand.

safe

Bare bottom (quarantine and grow-out only)Legitimate for quarantine and grow-out tanks. Easy to clean, but it holds far less biological filtration, and it is no long-term home for burrowing loaches or sand-sifters.

safe

With a caveat

Crushed coral or aragoniteDeliberately raises hardness and pH. Correct for rift-lake cichlids, wrong for soft-water species.

with a caveat

Aquasoil for planted tanksExcellent for plants, but most active soils soften the water and pull pH and KH down for months, and many leach ammonia for the first weeks. Right for soft-water species; wrong for hard-water animals (livebearers, rainbowfish, goldfish, and snails, whose shells erode in soft, acidic water) unless you test GH and KH and remineralise. Cycle around it; do not stock into it.

with a caveat

Very deep sand bedsAnaerobic pockets form below about 2 inches in still sand. Either keep it shallow or keep it stirred.

with a caveat

Painted or coated gravelUsually fine, but coatings chip. Test a handful in a jar for a week first.

with a caveat

Never

Sharp or angular gravel with bottom-dwellersWears down barbels and leads to infection in corydoras and loaches.

never

Anything containing metal, limestone of unknown origin, or shells in a soft-water tankSlowly alters chemistry in ways that are hard to diagnose later.

never

This is our general list for freshwater fish. We have not built one specific to Goldfish yet.

Top resources for more info

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