Im a bit confused. Yesterday I got a new stealth heater for a 15 gallon tank. It is 50 watts, and set on 78 degress. I have a coralife thermometer, and no matter where I move It, it reads 75.6 degrees. I know both the heater and the themometer say they are accurate within plus or minus 1 degree, but it just does not seem right to me any suggestions.