My betta has severe fin rot so I moved him to a 1 gallon QT tank of bottled water. It got down to 72 or 73 now which is way to cold for a betta especially when he's trying to heal. The smallest heater that I have is 8 watts and it's for a 2-5 gallon tank. I'm really worried that if I put it in, it will overheat him.
What can I do to keep him warm? Should I go to petsmart and try to buy a tiny heater or should I try out the 8 watt heater. When I used the 8 watt heater before I put him in, it brought the temperature from 74 to 78 in 10 minutes. I don't know how hot it will go.