Bettas are tropical fish, and thrive in conditions from 78–80 degrees Fahrenheit. The minimum ideal temp is 74 degrees. 70 degrees is okay, but the cold water will cause a significant decrease in activity, and your fish will be less interesting and happy overall. read more
The proper water temperature for bettas is warmer than for most tropical fish-- about 80 to 85 F. Some sources will give lower temperatures, but these represent compromises between the needs of bettas and the tendency to keep them in unheated tanks or bowls. read more
Before putting your fish in a heated tank, be sure to let the heater sit for about 2 days to make sure the temperature is the same throughout the tank, it also keeps you from cooking your betta! Also, you should have a filter on heated tanks to keep one side from being 60 degrees, and the other 160 degress (its a bit exaggerated but fish don't do well with temperature change, and shock can kill them). read more