Guppies are tropical fish and therefore requires a warm water temperature in the range of of 70-78 degrees Fahrenheit or 20-26 degrees Celsius to replicate tropical water conditions. If your fish tank temperature is lower or higher than these, then you should consider installing a heater or cooler respectively for keeping your guppy fish stable and healthy.
You can easily track temperatures of the fish tank all day long to see the variations in the temperature and then install a heater or cooler accordingly. The best thing is to use a automatic climate control heater that can easily maintain the required temperature all day long. What i will also suggest is to use a heater with digital display of temperature so that you can also view the current temperature of tank yourself. Also you can install a manual thermometer as a backup in case of heater failure.
How to create ocean like artificial temperature conditions for Guppies ?
Guppies are tropical fish and therefore requires warm conditions. Ocean temperatures rises in the day due to sunlight and falls in night. These variations in the natural condition are responsible for fishes to decide whether its day or night. To create such conditions in a fish tank one can install a automatic climate control with a timer so that it can vary temperatures as per time. Ultimately we can make guppy fish fool as if it is living in ocean.