temperature control of water

Question
hi there,  I recently bought a 20 gal tank and I have a rena cal top light 100w heater, penguin 200 bio wheel filter, and six fancy tailed guppies.  I do a 50% water change daily as the tank is cycling (I had no idea about cycling because the guy in the store never mentioned it)and I was told that it would be best for the fish.  
Although I set my heater to 76c it keeps a constant temp of 80.7 or so. Even if I unplug it and let the temp drop to 76 it heats it back up again.  Its not hot in our apt. and I have already been to the store and made an exchange for a different one but get the smae results.  Have you come across this prob before??

Answer
Maybe your thermometer is wrong, or you just got a bad brand of heater.

I had this exact problem with the AquaClear line of heaters. They're good, you just have to set them at, say, 72 if you want them at 76. Look into the Stealth line of heaters if you want real quality.