Coldwater fish, typically goldfish, can tolerate low temperatures both indoors and outdoors in a pond. An indoor fish tank will usually regulate to the ambient temperature of the room it occupies, and will rarely, if ever, drop below 12 degrees, even in winter.
This is absolutely fine for keeping most species of goldfish, and will not usually require a thermostat to heat the water. It is always wise, however to purchase a thermometer, to keep an eye on the tank temperature.
The temperature of a coldwater tank for common goldfish and the vast majority of coldwater stock should not exceed 20°C ideally. Common goldfish will tolerate termperatures up to 22°C without any issues.
If you are concerned that the temperature of your coldwater aquarium will drop below 12°C, for example if it is located in an outbuilding or garage, purchasing a heater to maintain a temperature between 16-20°C might be a good idea.
Fantails and other fancy goldfish are often tank-bred, and will thrive in slightly higher temperatures than common goldfish. Each species will have its recommended temperature range, but maintaining a tank temperature of 18-22°C is usually advised, and an aquarium heater the ideal tool for the task.
Choosing the correct heater for your aquarium >>>
When selecting the correct size heater for your aquarium use the formula 1 watt per litre of water. For example, if you have a 50 litre aquarium, choose a 50w heater. If you have a 300 litre aquarium, choose a 300w heater, and so on.
Always round up and over-engineer your equipment. For example, if you have a 120 litre aquarium, choose the next equivalent heater upwards in scale - 150w.
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