How is your Betta acting to the water temp...any change in behavior.....
So true....the slow, gradual temp changes are generally tolerated by a healthy Betta and not only do you have those gradual day and night changes but you also can have different temp in the tank itself in different areas/level-especially unfiltered tanks...top-bottom-far corner away from the heater, under things especially live plants...a healthy Betta can tolerate this generally without problems....but that sick, stressed, compromised Betta might have a problem and sudden big temp swings over 20-30 degree can stress even a healthy Betta and could send a sick one over the edge.......
This year in my outside spawns- my water temp has gone way over 100F and are tolerating it fine and reproducing like rabbits.....lol.....but-it also cools down some as the sun goes down....I don't really like to compare too much with native habitat with some things..... but only because we are talking about different volumes of water....no matter how big our tank- it just doesn't compare and the Bettas we keep are domesticated or man made in regards to finnage.......
Personally, I like to keep my water temp in the low range of normal...in the 75-76F range...and 80F is fine-but you also have to watch the Betta...each Betta can be different in their tolerance to water temp for varied reasons......