Since Bettas typically live in tropical conditions year-round, be sure to have a heater to keep them warm. The shallow channels and ponds they live in can change in temperature quickly, so bettas are much more tolerant of cold than most tropical fish.
Bettas have very strict needs when it comes to the ideal temperature. Your ideal temperature is between 78-80°F. However, they can survive in temperatures as high as 85°F and as low as 22°C.
In summary, keeping your betta fish in cold water will not die instantly. However, since you are depriving them of their ideal living ecosystem, they will not stay healthy or survive more than 6 months. As a tropical fish, they become susceptible to various life-threatening diseases when kept in cold water.
If you are planning a water change for a heated aquarium, you must bring your water temperature up to that of the aquarium water. Heat a small portion of your new water and slowly add it to room temperature water to bring it to the right temperature. Never fill boiling water directly into your aquarium!
The temperature should be kept at 75-80 degrees Fahrenheit. Colder water can kill the betta as the betta’s immune system slows down and makes it more susceptible to disease. Hotter water can make them uncomfortable and age them quickly as their metabolism increases. The temperature should be maintained with a heater.
Additional ways to cool your aquarium water
One way to make your aquarium warmer is to raise the temperature in the room where the aquarium is located by heating it to 78°F (25.5°C). Keep the aquarium in a warmer area of your home or closer to a heater.
One of the best ways to tell if your betta is going through thermal shock is to check the tank’s water temperature with a reliable thermometer. If the water temperature is drastically outside of the ideal range (+/- 10 degrees), you are most likely dealing with a sick Betta.
Betta will stop eating in water that is too cold, and they may not have enough oxygen in very cold water. Your fish quickly become lethargic in cold water and may hover near the bottom of the tank trying to warm up.
Increased water movement is the quickest way to increase the oxygen (O2) levels in an aquarium as it allows more O2 to be dissolved and carbon dioxide (CO2) to be released. This can easily be done with an air pump, large water changes, stirring the water manually, or placing a fan near the aquarium.
Yes, as long as you have a heater the temperature shouldn’t fluctuate. I would be careful not to place the tank directly in front of the airflow (especially if you don’t have a hood on your aquarium) to minimize evaporation. Also, do not place the aquarium in front of a window or in direct sunlight.