Do I need a heater for my betta?

I have a veiltail betta fish and I don't have a heater for him. His tank is always between 72-74 degrees f. I read online that they like it to be around 78 degrees. Is it bad to keep his tank between 72-74? Or should I buy a small heater? If so then what kind would be best for a 2.5 gallon tank?
Update: In response to Elliot. I did do my research, I'm a college student though I'm not going to go out and spend a ton of money on a tank. He's not on the verge of dying at all, he swims around happily and is always exited to eat. This is why I was debating the heater or not. I bought him a filter even... show more In response to Elliot. I did do my research, I'm a college student though I'm not going to go out and spend a ton of money on a tank. He's not on the verge of dying at all, he swims around happily and is always exited to eat. This is why I was debating the heater or not. I bought him a filter even though the pet store suggested not to because multiple places suggested it. I'm gonna go with my instincts on this and say he'll be fine without the heater. I've had him for about three months and he seems to be doing better than average.
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