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First it would depend on the size of your tank. Goldfish should not be kept in bowls. They require at least three gallons of water for every inch in length. Also they need lots of filtration. To start you should add a de-chlorinater and a probiotic to the water. For water changes there are two schools of thought on the matter. Some will say to change 10% of the water every week. Others will say to change 25% every month. Both ways work, it's up to you. Make sure that when you do water changes that you use water that is the same temp as your aquarium and that you treat it as wellFor more info, I would siggest buying the goldfish handbook from you local pet store.