Goldfish
Carassius auratus
Min Tank Size
110L
Adult Size
30.5 cm
Lifespan
15 years
About
Goldfish have been selectively bred for over a thousand years, originating in ancient China from wild carp. Today there are two broad camps: single-tailed varieties like Comets and Commons, which are genuinely pond animals capable of reaching 30+ cm, and fancy double-tailed varieties like Orandas, Ranchus, Ryukins, and Telescope-eyed fish, which are far better suited to aquarium life but still demand serious space. No goldfish is a small-tank fish, full stop.
A single fancy variety needs at least 75 liters to itself, with an additional 40 liters per additional fish as a practical minimum. They're coldwater fish, thriving between 10 and 24 degrees Celsius without a heater, which immediately rules out most tropical community setups. Water chemistry should lean slightly hard and alkaline, with good buffering capacity to prevent pH swings. Filtration needs to be aggressive since goldfish produce extraordinary amounts of waste. A filter rated for double your tank volume is a reasonable baseline.
Feeding is straightforward but overfeeding is a chronic problem, contributing to both water quality crashes and swim bladder issues in fancy varieties. A quality sinking pellet or gel food supplemented with blanched vegetables and occasional protein is a solid routine.
Despite all the caveats, goldfish genuinely reward attentive keepers. They recognize their owners, respond to feeding cues, and develop distinct personalities over time. Fancy varieties in particular develop striking color, flowing finnage, and head growth (called a wen) that make them genuinely spectacular display fish. If you want to see how impressive a well-kept goldfish setup can look, browse community tank builds featuring this species.
Water Parameters
Temperature
°CpH
GH
dGHKH
dKHSwimming Level
Flow Preference
Compatibility
Goldfish are best kept with other goldfish. Mixing single-tailed and fancy varieties is generally a bad idea since the faster, hardier single-tails outcompete the fancies for food. White Cloud Mountain Minnows are one of the more commonly cited coldwater companions and can work in larger tanks. Weather Loaches and Hillstream Loaches also tolerate coldwater conditions and can coexist reasonably well. Tropical fish are not suitable, the temperature overlap is too narrow to trust long-term. Shrimp will be eaten. Large snails like Nerites may survive but small ones are at risk. Avoid any fin-nipping species since fancy varieties with long flowing fins are easy targets.
Commonly kept with
Species this one is most often paired withCommonly tried but avoid
Often paired, but shouldn't beCare Notes
The single biggest mistake beginners make is starting too small. A bowl or a 40-liter tank is a death sentence for this species, just a slow one. The second biggest mistake is under-filtering. Goldfish produce more waste than almost any other commonly kept freshwater fish, and ammonia spikes in understocked filtration are a leading cause of disease. Fancy varieties are also prone to constipation and swim bladder problems when fed dry foods exclusively. Soaking pellets before feeding, offering gel food, and fasting one day per week all help. A tight-fitting lid matters too since goldfish can and do jump.
Behavior & Aggression
Goldfish are not aggressive in any meaningful sense. They don't defend territories, fight conspecifics, or bully smaller fish intentionally. The real hazard is their size and feeding behavior. A large goldfish will vacuum up anything small enough to fit in its mouth, including small tankmates, not out of aggression but out of opportunistic feeding. Fancy varieties with poor eyesight, particularly Telescopes and Bubble Eyes, can struggle to compete for food with faster, more agile fish, which creates welfare concerns even without any direct aggression.
Things to Know
- Comet and Common varieties reach 30+ cm and belong in ponds, not aquariums
- Massive waste producers, require oversized filtration and frequent water changes
- Will eat or uproot most live plants
- Do not keep with tropical fish, temperature ranges conflict
- Fancy varieties (Oranda, Ranchu, etc.) are prone to swim bladder issues
Community Sightings
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