In most homes, guppies need a thermostat-controlled aquarium heater. Domestic guppies generally suit a steady temperature around 22–26°C (72–79°F), with about 24°C (75°F) a reasonable starting point. An unheated tank is only an option if its water reliably stays suitable throughout the day, night and changing seasons.
The important word is water. A room that feels comfortable to you in the afternoon may become quite a different place after the heating switches off overnight.
Check the aquarium, not the weather forecast
Use a separate aquarium thermometer and look at the temperature at different times, including early morning. One warm reading doesn't show you what happens during the coldest part of the night.
A warm climate or heated fish room may keep an aquarium within a suitable range without an individual heater. That is different from assuming that the living room always feels reasonably warm.
For most ordinary homes, a thermostatic heater is the straightforward choice. It switches on when needed rather than heating continuously, provided it is working correctly.
What temperature should you choose?
Around 24°C is a useful starting setting for a typical domestic guppy tank. Keep the fish's previous conditions and any tank mates in mind, and check the actual reading rather than trusting the dial alone.
You don't need to chase every half-degree movement. Consistency within an appropriate range matters more than repeatedly adjusting equipment to hit an exact number.
Warmer is not automatically better. Higher temperatures affect metabolism and reduce the amount of oxygen water can hold. Keeping the tank at the top of a suggested range simply to encourage faster breeding is not my recommendation for an everyday display.
Choose and position the heater properly
Select a heater rated for your aquarium volume and the difference between room and desired water temperature. Follow the manufacturer's guidance on positioning, submersion and installation.
Place it where water circulation can distribute the heat. Don't hide it in a stagnant corner simply because that makes it less visible from the sofa.
A separate thermometer is still useful with an expensive heater. The dial is a setting, not independent proof that the water has reached it. Check both heating and filtration as part of your normal daily look at the fish.
What if the aquarium becomes too cold?
Check whether the heater has power and whether its setting or position has changed. If it has failed, replace it with suitable equipment and allow the temperature to recover gradually.
Don't pour hot water into the aquarium or make a large, sudden adjustment. Avoid creating a second problem whilst trying to solve the first. Fish that are very weak or breathing abnormally need prompt attention to the whole aquarium, including water quality.
If the temperature keeps falling whenever the room cools, reconsider heater capacity and tank location rather than repeatedly turning the dial higher.
What if it becomes too warm?
A heater cannot cool water. Check for a stuck-on heater, direct sunlight or unusually warm room conditions. If equipment is overheating the tank, switch it off safely and follow the manufacturer's instructions before handling it.
Improve surface movement and bring the temperature down gradually. Don't tip ice or very cold water directly into the tank. Persistent gasping warrants the checks in our surface-breathing guide.
Temperature works alongside oxygen, filtration and water chemistry. In Guppies, I explain how those parts fit together, so buying a heater is one informed decision within a setup that suits your fish.
