Well, you probably have a malfunctioning thermometer. I mean, technically, I don't think the heater make sense right now. Anyway, you could just bring it back to the petstore, they'll give you a return. You always want to have a working thermometer because, well, a broken one just wouldn't work. Good luck fixing your thermometer dilemma!
Just went to fish store and they did not have any thermometers...
but, if you check the temp with your fingers it is quite warm..
I think it IS ACTUALLY 79-80F in the tank
I have the Minibow 2.5g with 15w incandesant bulb with hood, so, maybe it is actually heating the water.
This morning while the light was out, the temp was still about 78f.
My daughter's tank in her room stays at 80* with no heater. It's a 2 gallon. Yesterday, the air conditioner was on, and the tank stayed at 80. I verified it with a meat thermometer that I keep around for that purpose.
I'm not sure how her tank holds at 80 when the house is cooler, unless the filter is keeping the water warm with the impeller (even after a night of darkness, the temp is 80, so I don't think it's the light) ... but when I put the heater into her tank, the temp shoots up to 85 or higher, so I just took it out. Maybe in the summer when we run the A/C full time, the heater will need to go back in. For now, the house is at 73, and her tank is at 80, and whatever ... I verified it so I'm happy.
Test it with a different thermometer just to be sure, but chances are you're fine.