The sticker type thermometers are not very accurate and tend to be affected by external temperatures. To better monitor your tank water temperature, you can buy a thermometer that sticks on the inside with suction cups
A heater can raise the temperature of a fish tank if needed
about 85F
Normal tropical fish tank temperature; 77F-82F
Inside the tank as it is for you to see the water temperature.
Yes it is important to have a thermometer in your fish tank. The temperature of the water should be between 75-80 degrees.
So you can make sure the fish has a suitable temperature
It is used to regulate the temperature of the water
The instrument that is needed to measure the temperature of the water is called an "Aquarium Thermometer".
If the temperature is around 20 degrees (Celcius) probably not. But for a five GALLON fish tank, probably.
Typically heaters are needed for tropical fish tanks. However, if you could guarantee the room temperature where the tank is located will be maintained at the needed temperature for your fish, then you don't need a heater. Most tropical tanks need to be maintained at temperatures of 72 to 80 degrees Fahrenheit, depending on the types of fish in the tank.
I've had many freshwater fish and room temperature seems to work. with tropical fish you may need a heater but not with goldfish.
To prevent shock which could kill your fish.