I bought my male betta fish Pablo around April of this year. I never used a heater with him because I couldn't find one to put in my 2.5 gallon tank. Now I've had him for roughly three months and still haven't used a heater. I just transferred him into a ten gallon tank because he seemed a little bored and clausterphobic in his other tank. His new tank also has a filter which I never used in his other tank.
My question is, should I buy a heater? I still have yet to use one with him and he seems relatively healthy. I mean, I know buying a heater would be best for his health, but can I get by with not using one for a few weeks?
And, if I put a heater in, could he get sick from the change in temp? He's used to cooler water now so I'm not sure if warming up his tank all of a sudden would be good for him.