Goldfish (Fancy) Carassius auratus

Quick Stats

Tank Setup

Fancy goldfish (Carassius auratus, double-tailed breeds like ranchu, oranda, ryukin, fantail, telescope) are the largest commonly-kept cold-water aquarium fish. The 'one inch of fish per gallon' rule does NOT apply — a fancy goldfish produces roughly 5× the waste of a similarly-sized tropical fish. The realistic minimum is 20 gallons for the first fish and 10 gallons per additional fish. A 30 gallon tank comfortably houses two fancies for life.

Water parameters: temperature 15–22°C (unheated in a normal house is ideal — they do NOT need a heater and suffer above 24°C), pH 7.0–8.0, hardness 10–25 dGH. They prefer harder, alkaline water. Soft acidic water damages their slime coat over time.

Filtration is the non-negotiable. A fancy goldfish produces enormous waste — plan on a filter rated for 3–4× the tank volume (a filter rated for 90–120 gallons on a 30 gallon tank is appropriate). A canister filter or a large hang-on-back with a pre-filter sponge is the standard setup. Strong surface agitation keeps oxygen high; goldfish are oxygen-hungry fish. Substrate should be sand or smooth gravel — they constantly root through it looking for food, and sharp gravel damages their mouth and barbels.

Tank Mates

Fancy goldfish are peaceful but have two limitations as tank mates: temperature (cold-water, so most tropical fish are out) and predation (they will eat anything that fits in their surprisingly large mouth). The safest tank mates are other fancy goldfish of the same body type.

Compatible tank mates: other Fancy Goldfish of the SAME body type (don't mix single-tail commons with double-tail fancies — commons are faster and outcompete fancies for food), White Cloud Mountain Minnows (temperature overlap), Weather Loaches, large Cherry Shrimp (small shrimp will be eaten), and Japanese Trapdoor Snails. Do NOT house with tropical fish that need 24°C+.

Avoid: tropical fish (temperature mismatch), small fish that fit in a goldfish mouth (neon tetras, guppies, endlers), fin-nippers (tiger barbs, serpae tetras — they damage the fancy's flowing fins), and aggressive bottom-dwellers (cichlids, large catfish). Common/single-tail goldfish should NOT be housed with fancies — the commons outcompete them at feeding time and the fancies slowly starve.

Diet & Feeding

Fancy goldfish are omnivores with a near-constant appetite. The staple diet should be a high-quality sinking pellet (soak pellets for 2 minutes before feeding so they don't float and cause the goldfish to gulp air, which leads to swim bladder issues). Repashy Super Gold, Hikari Saki, and Omega One are widely used brands.

Feed once or twice daily — only what they can consume in 2 minutes. Fancy goldfish will literally eat themselves to death if given the chance, and overfeeding is the #1 cause of swim bladder disorder. A rested digestive tract (one fasting day per week) prevents most digestive issues.

Supplement with blanched vegetables (peas with the shell removed are a classic laxative, plus zucchini, spinach, cucumber) and occasional frozen brine shrimp or bloodworms. Avoid floating flakes — they encourage surface gulping and the fish ingest air, which lodges in the swim bladder and causes the fish to flip upside down. Sinking foods only.

Common Health Issues

The two health issues that dominate fancy goldfish keeping are swim bladder disorder and digestive impaction. Both are usually caused by overfeeding, dry food, or water that's too warm. Symptoms include floating upside-down, sinking to the bottom, or swimming at an angle. The fix is usually: fast the fish for 3 days, then feed blanched peas (a natural laxative) for 2 days. If that doesn't resolve it, raise the temperature to 26°C briefly and treat with Epsom salt (1 tbsp per 5 gallons).

Goldfish are also prone to ich (white spot) when stressed by temperature swings. The standard heat-and-salt treatment works well for goldfish — they tolerate salt at 1–3 tsp per gallon, much higher than most tropical fish.

Lifespan is 10–15+ years in a properly set up tank. A fancy goldfish that dies in 2–3 years was almost always kept in a tank that was too small or fed incorrectly. The oldest recorded goldfish lived to 43.

Breeding

Fancy goldfish are egg-scatterers and breed readily in cold-water setups. They spawn in the spring, triggered by a temperature rise (from 15°C to 20°C) and a water change with slightly cooler water.

Sexing: males develop small white bumps on their gill covers (called breeding tubercles) and chase females. Females develop a rounder, asymmetrical belly when full of eggs. To breed deliberately, condition a group (2 males to 1 female) with high-protein food for 2 weeks, then do a 50% water change with slightly cooler water. Spawning happens at dawn — the fish chase each other frantically and scatter thousands of adhesive eggs over plants and substrate.

Remove the adults immediately after spawning — they will eat every egg. Eggs hatch in 4–7 days depending on temperature. Feed fry infusoria for the first week, then baby brine shrimp and crushed flake. Culling is unavoidable — you will have hundreds of fry, most with deformities or poor body shape. Ethical breeding means being prepared to cull humanely or rehome hundreds of juveniles.

Frequently Asked Questions