Heater burnt my betta :(

[Aquagirl]

I recently got a new Betta named Samari (about a month ago) and I bought him a little heater from Wal-mart, to keep the temp up around 78-80 and he was seemed really happy with it but this morning I looked at him and noticed a burn mark on his little face near his gill :(. I think he tried to swim behind it which I've seen him to before and it must have got him, so my question is how can I prevent Samari from getting hurt but still keep the tank nice and toasty?

[betta mom]

i have heaters for all my betta's , i've never had any trouble . thats wierd he got burnt . what size tank do you have and what size heater ? i have my betta's each in 2.5gal tanks and heaters are for 2-5 gal .:22:

[Aquagirl]

I also have a 2.5 gallon tank and a heater for tanks 2 to 5 gallons. I've had heaters in other tanks with no problems but this is my first Betta tank.