As often as required. It's hard to say as everyone's tank is different. But if it smells then it probably needs changing. If you feed too much or don't have a filter fitted to the tank then you'll need to change some of the water daily. Don't change all the water or you may kill your creatures. Change about 25% maximum a day. Make sure the new water is kept at the same temperature as the tank water. Sudden water temperature changes could harm your aquatic pet.