Comet Goldfish
Carassius auratus
A fast, hardy single-tailed goldfish best suited to ponds and very large tanks.
| Difficulty | Beginner |
| Min. Tank Size | 200 L |
| Temperature | 15.0–24.0 °C |
| pH Range | 7.0–8.4 |
| Max Size | 30.0 cm |
| Lifespan | 10-15 years |
| Diet | Omnivore |
| Temperament | Peaceful |
Overview
The Comet Goldfish (Carassius auratus) is a fast, hardy single-tailed goldfish best suited to ponds and very large tanks. It is a popular choice among Australian aquarists, reaching around 30 cm and living roughly 10-15 years when properly cared for.
This complete guide covers how to keep the Comet Goldfish successfully - tank size and setup, water parameters, the best foods (including recommended brands), suitable tank mates, breeding, sexing and the health issues to watch for. It is rated Beginner to care for.
Natural Habitat & Origin
Goldfish are a domesticated coldwater carp from East Asia. They are not tropical fish - they prefer cooler water, produce a heavy bioload, and need large, well-filtered tanks or ponds rather than bowls.
Matching the temperature, water chemistry and cover a species evolved with is the simplest route to keeping it healthy and seeing its natural behaviour and colour.
Tank Size & Aquarium Setup
Provide a minimum of 200 litres for the Comet Goldfish. As a group-living species, swimming length and floor space matter more than height.
Use strong filtration and never a bowl: goldfish are messy, heavy-waste fish that need large, well-filtered tanks or ponds. Most fancies do best at 18-23°C with no heater.
Always add fish only to a fully cycled, mature tank with stable biological filtration. Match filtration generously to the fish's size and waste output.
Water Parameters
Keep the Comet Goldfish in stable water at 15-24 °C with a pH of 7.0-8.4. It adapts to a moderate range, so stability matters more than an exact figure.
Test regularly: ammonia and nitrite must read zero, and nitrate should be kept low with routine partial water changes. Always dechlorinate and temperature-match new water, because sudden swings cause far more illness than water that is stable but slightly imperfect.
Diet & Feeding
The Comet Goldfish is an easy-going omnivore that accepts a wide range of foods.
In Australia, good options include a quality goldfish staple such as Hikari Saki-Hikari or API Goldfish Pellets, plus blanched peas and vegetables to prevent constipation. Feed small amounts once or twice a day (only what is cleared in a minute or two), vary the diet for the best colour and health, and avoid overfeeding, which is the leading cause of poor water quality.
Temperament & Tank Mates
The Comet Goldfish is peaceful and community-friendly. It mixes well with other calm species that share its temperature (15-24 °C) and pH (7.0-8.4) needs, are too large to be eaten and not large enough to eat it.
It is happiest in a group of its own kind and is shyer and less settled when kept alone.
Breeding
The Comet Goldfish is an egg-scattering species. To breed it, condition a group on rich foods, then move them to a dim tank with fine-leaved plants or a spawning mop; the parents scatter eggs and will eat them, so remove the adults after spawning.
Even if you are not planning to breed it, recognising this behaviour helps you understand what you are seeing and respond well - for example by adding cover for fry or giving a guarding pair extra space.
How to Tell Males from Females
Sexing is only reliable in breeding season, when males develop small white breeding tubercles on the gill covers and pectoral fins, and females swell with eggs.
Common Health Problems
Goldfish are prone to swim bladder disorder (especially round-bodied fancies), constipation, ich and fin rot. Feed sinking, soaked or vegetable foods and keep water clean to avoid most problems. Quarantine new arrivals for 2-4 weeks and watch daily for early signs such as loss of appetite, unusual hiding, clamped fins or laboured breathing - caught early, most issues are very treatable.
Prevention beats cure: keep water pristine, avoid overstocking and overfeeding, quarantine new arrivals, and act at the first sign of trouble.
Is the Comet Goldfish Right for You?
The Comet Goldfish suits beginners and experienced keepers alike, as long as you can provide at least 200 litres, water at 15-24 °C and pH 7.0-8.4, the right diet and a proper group of its own kind.
Meet those needs and the Comet Goldfish will reward you for around 10-15 years. Use our free aquarium calculators to plan your setup, and explore our other fish, plant and disease guides to build a thriving aquarium.