Moderate-high (heavy waste, frequent water changes)
Cost
Medium
Overview
The common goldfish is a coldwater fish descended from wild Asian carp, not a disposable bowl fish.
Kept properly, single-tail goldfish reach 20-30cm and commonly live past 15 years, with pond fish sometimes exceeding 25.
They are hardy and personable, but their adult size and heavy waste output make them more demanding than their low price suggests.
They belong in large aquaria or, ideally, garden ponds.
Housing
Goldfish are coldwater fish that thrive at roughly 18-22C with no heater needed; single-tails do well in unheated rooms or outdoor ponds and tolerate seasonal cooling.
Provide a fully cycled tank of at least 280L for one fish, adding around 75L per additional goldfish, with strong filtration sized above the rated load.
Keep ammonia and nitrite at zero and nitrate below 40ppm; a pH of 7.0-8.0 suits them.
Avoid sharp decor, do weekly 30-50% water changes, and never use a bowl, which stunts and kills them.
Diet
Goldfish are omnivores prone to obesity and constipation, so feed a quality sinking goldfish pellet as the staple plus plenty of blanched vegetables such as peas, spinach and courgette.
Pre-soaking dry food helps, as gulping air at the surface can contribute to swim-bladder problems.
Feed small amounts once or twice daily, only what they finish in a couple of minutes.
Avoid high-protein tropical foods and overfeeding, the leading cause of disease and fouled water in this heavy-waste species.
Health
Goldfish are hardy but pollution-sensitive, and commonly suffer ammonia poisoning, fin rot, ich and swim-bladder problems, nearly all traceable to undersized tanks, poor filtration or overfeeding.
Watch for clamped fins, gasping at the surface, or floating and sinking abnormally.
Maintain clean water and stable temperature to prevent most issues.
Quarantine new fish for 2-4 weeks, and treat ich with appropriate medication.
A varied diet and roomy housing keep them healthy for many years.
Temperament
Goldfish are peaceful, curious and trainable, learning to beg at feeding time and recognise their keepers.
They are social and do best in small groups of their own kind rather than alone.
Keep them only with other goldfish or similarly cool, slow swimmers; they will outcompete or eat tiny tankmates and are too messy for delicate community fish.
Mixing fast single-tails with slow fancy varieties is unwise, as commons outcompete fancies for food.
A good fit for
Pond keepers with outdoor space
Patient owners wanting a long-lived pet
Those willing to over-filter a large tank
Keepers who prefer coldwater over tropical setups
Common mistakes to avoid
Bowls and tiny tanks that stunt and kill
Overfeeding causing fouled water and constipation
Underestimating adult size and 15+ year lifespan
Mixing with delicate or tiny community fish
Is your enclosure big enough?
Minimum volume280 litres (74 US gallons)
CompanyCannot be kept alone. The minimum is 2, and the space above is for that group.
In a fish tank the patient is usually the water. Test before you treat — dosing a healthy tank for a disease it does not have makes things worse.
Go now
Any detectable ammonia or nitrite in a stocked tankLarge water change now with dechlorinated, temperature-matched water. Stop feeding. Test again in 12 hours. Both are directly toxic. How toxic the ammonia reading is depends on pH and temperature, so the same number can be harmless or lethal.
Now
Fish gasping at the surface, or crowding the filter outflowIncrease surface agitation immediately, then test the water. Either the oxygen has crashed or the gills are being chemically burned. Both are hours-scale problems.
Now
Sudden deaths, more than one fish in a dayTest ammonia, nitrite, nitrate and temperature before doing anything else. Do not add medication yet. Multiple simultaneous deaths point at the water or the equipment, not at a pathogen.
Now
Today
Flashing — repeatedly scraping against decor or substrateTest the water, then look closely for parasites. Irritation is either chemical or parasitic; the test tells you which before you medicate.
Today
White salt-grain spots over the body and finsBegin the appropriate treatment today and check the temperature. The parasite's life cycle is temperature-driven, and it multiplies with every cycle you wait.
Today
Soon
Clamped fins, hiding, or refusing food for more than two daysTest the water and review recent changes. Usually the earliest visible sign of a parameter drifting.
A few days
Never add a new fish to a tank that is not fully cycled, and never treat blind. A test kit is cheaper than the fish.
This is general guidance for a healthy adult, not a diagnosis. An exotics vet who knows your animal always overrides it — and the time to find one is before you need one.
What it really costs
Setup, monthly, first year and across a whole life, itemised and with the electricity worked out from the actual wattage. Published figures for the same animal disagree by a factor of four, so the calculator shows ranges rather than picking one.
Intermediate
Advanced
Beginner
Beginner
Beginner
Beginner