According to the Rochester Institute of Technology, a goldfish tank should be kept at room temperature. A heater is not needed. However, tap water should be allowed to sit overnight in order to reach the proper temperature, as well as to let chlorine evaporate.
Proper water conditions are essential for goldfish survival. Although most tap water is suitable, pet shops are typically aware of anything potentially harmful in the local water supply and can often offer a solution. Clean water is also necessary. At least once a week, one-third of the tank's water should be removed with a siphon tube and replaced with room temperature, de-chlorinated water. If many goldfish are kept together, debris builds up quickly on the tank's bottom. Therefore, the water must be changed more frequently.