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: Unknown / Wikimedia Commons · Source · CC-BY-SA-2.5