How Often Should You Change Goldfish Water?

Change goldfish tank water every 3-7 days, adjusting for season, temperature, stocking density, and water quality. In summer without filtration, change about a third of the water every 3 days; in winter you can stretch to 10-15 days. Keep the new-water temperature within 2°C of the old, add it slowly along the glass, and replace only 1/4-1/3 of the volume each time.