Bettas and Temperature

Hi. I last posted about what kind of small tank to get for my new betta. I
ended up with an Eclipse corner 5 tank (5 gal). I also picked up a 25 watt
heater as I was concerned about temp changes even though someone here said
his 3 gal didn't fluctuate at all. I guess the lfs guy talked me into it,
plus getting a heater seems the right thing to do anyway.

So, I hook everything up and find out that tank is running at about 83-84
degrees F. It was even this warm in the early morning before any lights
were on. I have to assume the motor is generating a lot of heat as the
apartment is only about 72. I know bettas like it warm (80?) but am I
getting into a problem with oxygen content and/or stress at this high a
temp? Also, if I want to put any other fish in there, I will have a problem
unless I get the temp down.

Oh, and the thermometer isn't even on. It was this warm before I put the
thermometer in.

Thanks for any input,
dwhite