last night we set up our 20 gallon tank. we bought a 8" x 8" Substrate heater from Exo Terra. we dicided to place the heater underneath on one side and turn it on with no substrate in the tank. we also had the cover on the tank. I know it says not to use this type of heater with out substrate but we have yet to get our BP yet so we wanted to see how warm we could get the tank. more of a test run of sorts. well they say that them type of heaters can burn a snake but it hardly warmed the bottom of the cage at all. it is a 4 watts and we couldnt get the overall temperature in the tank over 70 F andf our room temperature is about 68 - 70 F. so it never even warmed the cage. i dont see how this is going to heat the cage once we do get substrate in the cage. what is our problem? thanks