Goldfish are often treated as disposable pets, but with good care they can live 10 to 15 years or more, and some common goldfish live well past 20. Most problems come from tanks that are too small and water that is not properly filtered.
At a glance
| Lifespan | Often 10–15+ years |
| Adult size | Fancy varieties often 15–20 cm (6–8 in); common goldfish can grow much larger |
| Social needs | Social; do well in groups of goldfish |
| Water | Cool to room temperature, well filtered, with regular partial water changes |
Why bowls are unsuitable
Goldfish produce a lot of waste, which breaks down into ammonia, a substance that is toxic to fish. In a small bowl without a filter, ammonia builds up quickly, oxygen levels drop, and the fish becomes stressed and ill. Bowls also do not have room for a goldfish to grow to its adult size.
Tank and equipment
- Size: many fish-keeping guides suggest starting at around 75–100 liters (20–30 US gallons) for one or two fancy goldfish, with more space for each additional fish. Common and comet goldfish grow large and are better suited to very large tanks or ponds.
- Filter: choose a filter rated for a larger tank than yours. Goldfish are messy.
- Water conditioner: always use a dechlorinator when adding tap water.
- Test kit: to check ammonia, nitrite, nitrate and pH.
- Goldfish do not normally need a heater at room temperature, but avoid rapid temperature changes.
Cycling the tank
Before adding fish, a new tank needs to be “cycled”. This means growing beneficial bacteria in the filter that convert toxic ammonia into nitrite and then into less harmful nitrate. This process usually takes several weeks. Ask your aquarium store about fishless cycling, and test the water until ammonia and nitrite both read zero.
Diet
- Feed a quality goldfish pellet or flake once or twice a day, only as much as they eat in a couple of minutes.
- Add variety with blanched peas (skins removed) or leafy greens.
- Overfeeding is the most common mistake. Uneaten food pollutes the water.
Maintenance
- Change about 20–30% of the water weekly, using conditioned water of a similar temperature.
- Rinse filter media in old tank water, not tap water, so you do not kill the beneficial bacteria.
- Never replace all the water and clean all the filter media at once.
Signs of illness
- Gasping at the surface (often poor water quality or low oxygen)
- Clamped fins, lethargy, or lying on the bottom
- White spots, fuzzy patches or red streaks on the body or fins
- Trouble staying upright or swimming normally
Test the water first; many illnesses start with poor water quality. For persistent problems, consult an aquatic veterinarian or an experienced fish specialist.
Sources
- RSPCA fish care guidance
- Standard aquarium fish-keeping guidance on the nitrogen cycle