25 degrees is too cool for an olive. Just provide a warm basking spot at one end of the cage so the snake can regulate it's temperature by itself. People put far too much emphasis on cages temps etc, I've never checked the temperature in an enclosure for years. Look at the animal, if it's always trying to get away from the heat source then it's too warm, or if it never moves from under it it's probably too cold. Even in a 6 ft long cage all you need is a 40 0r 60 watt spot globe up one end and you'll have a happy and healthy snake. That way it can bask when it want's to and not have a set temperature forced upon it 24/7