A new aquarium will require several weeks to completely cycle. This is the period during which beneficial bacteria will build up and establishes themselves inside the aquarium. These bacteria helps to break down toxic ammonia produced by goldfish and decaying organic matters.
While you are waiting for the tank to cycle, i recommend you purchase a water test kit from the pet store to monitor the ammonia, nitrite and nitrate levels in the aquarium , since these levels can fluctuate wildly during this period and possibly harm your fish. The water testing will let you know more accurately when you should do a water change. Remember to purchase a water conditioner also in order to remove the chlorine and chloramine.
Once your aquarium is well established , you can do a 15- 20 % water change every two weeks, or as necessary as indicated by a water test.

hey u need to change water for 2days ones if u have filters and temtp controllers no need then you have to change for 15to20 days and dont forget to put some clorine in ur tank were u can get it from u r nearest pet shop..and some medicine.....