trouble regulating water temp.

Question
hello, I am having trouble keeping my tank at the right temp. I have a 100w rena cal top light heater in a 20 gal. tank w/ a penguin bio wheel filter and 6 fancy tailed guppies.  I do daily water changes because the tank is cycling and i was told this was best for the fish.
 I followed the instructions for the heater, set it to 76 and it runs perfectly at 80.7, or very close to it.  I then called the store and they exchanged it for me.  The new heater is exactly the same, if I set it to 74, 76 or 80 it runs at just over 80.
 I bought a good digital thermometer just to make sure I wasnt going nuts and it is not very hot in the apt.  Have you ever heard of such a thing and do you have any theories??

Answer
Hi Emmy,
Sounds like you have a powerful heater. Or maybe just an unreliable one. I've run into many of those types. The best most reliable heater I've ever had was VISA-THERM and VISA-THERM Stealth heaters. They seem to be most accurate. Another thing is- if you have one of those thermometers that sticks to the outside of the tank keep in mind those aren't -real- accurate. And the stick type that go inside the tank are much better and tend to be more accurate (place the thermometer on the opposite side of the tank to the heater. The outside sticker-type are OK for a general guide to how warm the tank is, but to get a little more accurate, the inside suction-cup devices are more accurate.

I hope this helps and best of luck!
Karen~