temperate

im planning to get a betta in a 2.5 planted tank but im keeping it in my room with AC, i turn my AC on sometime when its hot, im planning to get a heater but still... how the heck can i avoid temperature fluctuations in an AC room even with heater?

In the summer months the ambient temperature around my tank fluctuates a lot, the tank temperature goes from 75-77 degrees quite a bit. I’m wondering if this is okay for my fish? If not is there any way I can keep the temperature more steady?

You guys might not know this but I work for a major pet store in the UK (Pets at home), although I have learnt a lot about fish in my 5 months of working there I would still consider myself somewhat of a novice when it comes to fish keeping, this is why i'm constantly asking you guys for advice...