I had a beloved Betta fish a week ago that passed away yesterday morning. I then...
You did not mention the water temperature, only that you kept the light on during the day for warmth, but turned it off at night. Bettas are tropical fish that require warm water, preferably 74-82 degrees Fahrenheit. They will die at temperatures below 64-68 degrees. What is the temperature in your aquarium?
It could also be a problem with the water quality. Are you using a dechlorinator when changing the water? What is the ammonia, nitrite, and nitrate level in your aquarium?
PetSmart offers free water testing, so you can take a water sample in to the store to have it tested. If you don't have an aquarium thermometer or heater, you can get that as well. We also have free care guides on keeping Bettas, and also on setting up aquariums and maintaining proper water quality. Ask a PetSmart associate for these informative brochures.