Goldfish Care Guide

The goldfish (Carassius auratus) is a domesticated carp. It is the most popular cold-water pet fish, and the one most often kept wrongly: often bought small or won as a fairground prize, it can live 25 years and grow past 12 in (30 cm). It suits anyone who can run a large, well-filtered, unheated tank, but not a bowl or a desktop “starter kit”.

Care requirements

Need	What to provide
Enclosure	At least 60 L (16 gal) of water per goldfish, in a filtered tank at least 100 cm (40 in) long. Wide beats tall, for surface area (RSPCA; FishBase)
Temperature	50–70°F (10–21°C). No heater usually needed (RSPCA)
Lighting	8–10 hours of aquarium light a day, off at night. Keep the tank away from windows (RSPCA)
Diet	Goldfish-specific flakes or pellets as the staple, plus chopped peas and spinach. Brine shrimp or daphnia only in small amounts. Two or three feeds a day, as much as they eat in 2–5 minutes (RSPCA)
Lifespan	Up to 25 years. FishBase records a maximum of 41 (RSPCA; FishBase)
Social needs	Keep in groups. FishBase suggests 5 or more, so tank size must allow for every fish (FishBase)
Legal notes	No common restrictions; check local rules. Releasing goldfish into the wild spreads an invasive species (Pennsylvania Sea Grant)

Common and comet vs fancy goldfish

	Common and comet	Fancy (e.g. fantail, oranda, lionhead)
Body	Long, streamlined, close to the wild shape	Shortened, rounded bodies and exaggerated fins or heads
Hardiness	The hardiest, because they haven’t been weakened by selective breeding	More delicate and prone to inherited health problems
Swimming	Fast and active, so they need length to swim	Usually slower, so they can lose out at feeding time
Adult size	Can pass 12 in (30 cm)	Stay smaller, but still need far more than a bowl
Welfare view	Widely recommended	The RSPCA discourages breeds with exaggerated features

Sources: RSPCA; FishBase. If you keep both types together, watch that fancies get their share of food.

The trouble with bowls

The RSPCA lists four problems with bowls, jars and novelty tanks:

• they are far too small, and goldfish need at least 60 L of water each
• there’s no room for an effective filter
• the small surface area lets too little oxygen into the water
• temperature swings fast, which causes stress and can kill.

Goldfish produce a lot of waste. The RSPCA’s stocking guide is 2 litres of water per 0.5 cm of goldfish length, twice what it allows for tropical fish. A 15 cm (6 in) common goldfish works out at 60 L on that measure alone. FishBase recommends groups of five or more and a tank at least 100 cm long, so a proper goldfish setup is a large tank or a pond, not a small one.

Cold water and the nitrogen cycle

Goldfish do best between 50 and 70°F (10–21°C), so most homes don’t need a heater. If your room swings a lot, the RSPCA suggests a thermostatic heater set to the bottom of that range as a buffer. Keep the tank away from radiators and sunny windows, which can overheat small volumes quickly.

Filtration matters more than heating. Fish waste releases ammonia. A mature biological filter converts it to nitrite and then to the less harmful nitrate. Run the filter for at least two weeks before adding fish (“fishless cycling”), and wait until ammonia and nitrite have spiked and dropped back to zero. Then follow the RSPCA’s routine:

• change 10% of the water weekly with a siphon
• rinse filter media in the removed tank water, never under the tap
• dechlorinate all new water
• test ammonia and nitrite at least every two weeks.

If either reading is high, change half the water daily until it falls (RSPCA).

Gotchas

• Fairground and prize fish. The RSPCA says prize goldfish often die from suffocation or temperature shock on the way home, and usually go into an unprepared container. Set up and cycle the tank first.
• Tropical flakes. Their nutrient levels differ from goldfish food. Buy food made for goldfish (RSPCA).
• Overfeeding. Goldfish always look hungry. Uneaten food rots and causes ammonia spikes, and the RSPCA says it is better to underfeed than overfeed.
• Holiday feeding blocks. They release a lot of food at once that mostly rots. Use an automatic feeder checked daily, or a fish-sitter (RSPCA).
• Release into a pond or river. Goldfish released into the wild have spread to every US state except Alaska. They eat fish eggs and young fish, muddy the water and can carry diseases such as koi herpesvirus (Pennsylvania Sea Grant).

Avoid

• Bowls, vases, jars and small “fun” tanks
• Untreated tap water: chlorine harms fish and filter bacteria
• Washing filter sponges under the tap
• Tropical fish food as the main diet
• Placing the tank in direct sun or next to a radiator
• Cigarette smoke and aerosol sprays near the tank (RSPCA)
• Releasing unwanted fish anywhere outdoors. Rehome them through a retailer, another keeper or a rescue instead (Pennsylvania Sea Grant)

Signs to see a vet

When buying, the RSPCA advises checking for split fins, white spots, open sores and concave bellies. Treat the same signs in your own fish as a reason to test the water and seek help. Also act on gasping at the surface, which suggests low oxygen or high ammonia, and on a fish that stops eating.