this coming from an hvac specialist. You will get extreme heat under the tank due to the lack of air circulation directly under the tank. i have checked these temps just now to find that they are 10 degrees hotter under the tank than inside the tank. But if you don't want to tape the probe inside the tank then i have several ways to perminately attach them. I you are using a substrate such as wood chips(i use newpaper) the temp may be even greater due to the insulation factors of the wood. i would say you are right with the tape but i use medical tape to prevent any possible toxic problem in the tank.