Put the prob between the uth and the glass for best and most accurate readings.... as for your temp problems a uth will not heat the tank, it will oy heat what it touches and the air above it slightly, if using a uth you need to wother controll the ambient temps of the whole room or you need another source of heat for your tank, you can use a red heat bulb or blue but if you dont need the light for viewing your better off getting a ceramic heat emitter (che) instead of the colored heat bulbs, they last far longer.... for the light make sure the socket is ceramic if your gonna go the che route cause they can get very hot, also make sure your light has a dimmer switch as opposed to the classic on/off style. If you cant find a dimmable light you can buy a plug in dimmer at home depot or lowes