Cleaning your fish tank and changing 1/4 of the water frequently is essential for healthy fish. At a minimum it should be done once a month but may be performed weekly to provide an optimal environment for your goldfish.
Before perfoming a water change use an algae scraper to clean the inside walls of the tank. Then clean your fish tank with a gravel vacuum. The vacuum cleans the tank by sucking dirt and old food out of the gravel. The gravel vacuum also removes some water from the tank. Stop the vacuum when 1/4 of the water has been removed.
Before you add fresh water, use a thermometer to make sure the fresh water is the same temperature as the water in the tank. Add conditioner appropriately and to complete the water change add the water to the tank.