What happens next?

How to keep a betta's water warm
Install a heater in the tank.
You must provide warm water to maintain the health of your betta fish. To do this, you will need to get a heater for your aquarium.

OPTIONS: -The type of heater you need will vary, depending on how large your aquarium is. The two main types of heaters should be available at your local pet supply store.. -A single-quart aquarium heater works best. Traditional stainless steel can also work if you plan to keep your betta in a cooler, but you will need to check the temperature with a thermometer first.. -Betta fish also have high evaporation rates, which usually indicates that they are in desperate need of a heater. Some systems, like the washing machine, can withstand high evaporation rates.. -To install a heater, you will need to insert a thermometer into the tank until it is above 40 f (4 c) and test the water level regularly. You can buy a thermometer online or at most pet stores..
The type of heater you need will vary, depending on how large your aquarium is. The two main types of heaters should be available at your local pet supply store.