Species Profile
Common Goldfish
Carassius auratus
Goldfishpeaceful
Adult size
14″
Minimum tank
75 gal
Temperature
60–72°F
pH
6.5–8
Schooling
Solitary OK
Water level
All levels
Grows from juvenile
Typically sold at ~2″ and reaches 14″ over ~3 years. Plan tank size for the adult, not the fish at purchase.Diet
Pellets, flake, blanched veggies, frozen
Notes
A pond fish, not an aquarium fish. Grows to 12–18″. Massive bioload; will eat any tankmate small enough to swallow. Coldwater, incompatible with tropical species.
Tank Setup
Common goldfish belong in a POND, not an aquarium. If you must keep one indoors: 75gal absolute minimum for a single fish, 150gal+ for a pair. Coldwater (60–72°F). NO heater. Massive over-filtration is non-negotiable; goldfish are pollution machines and produce more waste than any other commonly-kept aquarium fish. Bare substrate or large smooth rocks, they swallow gravel. No real plants (they eat or uproot everything); silk or weighted plastic instead. Industrial-grade canister or sump filter rated for 2× the tank volume.
Behavior
Always hungry, always looking. Goldfish recognize their keeper, learn to take food from hands, and develop genuine personalities, they're famously interactive. Constantly rooting through substrate. Will eat ANYTHING that fits in their mouth: small fish, snails, shrimp, plants. Active swimmers (single-tail varieties especially), they need horizontal swimming room more than depth.
Breeding
Spawn in spring in ponds after a winter chill. In tanks: drop temperature gradually to ~55°F for 6 weeks (simulating winter), then raise to 68°F + add spawning mops. Males chase females; eggs scatter and stick to plants/mops. Remove parents IMMEDIATELY after spawning, they will eat every egg. Eggs hatch in 4–7 days; fry need infusoria then microworms then fine flake. Home goldfish breeding is rare; most fancy-goldfish breeders work outdoors in temperature-controlled ponds.
Health
Common issues: swim bladder disease (overfeeding floating food, fast 3 days then peas), ich (white spots, raise temp to 78°F for 10 days, NOT higher; coldwater fish stress in warm water), fin rot (poor water quality), 'flipover' from genetics or injury. Goldfish live 15–25 years with proper care. A 2-year-old goldfish in a small bowl is dead from chronic stress, not old age.
Frequently Asked
Can I keep a goldfish in a bowl?
No. A bowl is too small to filter the goldfish's waste, has too little surface area for oxygen exchange, and gives them no swimming room. The 'pet store goldfish in a bowl' is a major reason goldfish have a reputation for short lifespans, they're dying of chronic stress, not old age. Same fish in a 75gal tank lives 15+ years.
How big do common goldfish actually get?
12–18 inches as adults, 2–3 pounds in a pond. Even in cramped tanks they stunt to 8–10" rather than staying small, that 'small goldfish' is a sick goldfish.
Can goldfish live with tropical fish?
No. Goldfish need 60–72°F coldwater; tropical fish need 75°F+. A compromise temperature kills both. The compatibility checker flags this as a critical mismatch. Goldfish coexist only with other coldwater species: White Cloud Mountain Minnows, Hillstream Loaches, paradise fish (with notes).
How long do goldfish live?
15–25 years in a properly-sized tank or pond. The oldest verified goldfish lived to 43. The 2–3 year lifespan people associate with goldfish is the result of inadequate housing (bowls, small tanks).
Photo: James St. John / Wikimedia Commons · Source · CC-BY-2.0