ok so on monday i am getting a 30 gal tank for george one of my main concerns is for heat since i have ac what wattage of bulb should i get and what kind my local pet store has too many to choose from

ive narrowed it down to a blue incandesant daylight bulb that doubles as a heat lamp(i dunno that is what it says on the box light&heat)
or a 100 watt spotlight bulb im not sure which would work best so if everyone can post what orks best for them with a description then i can know what to get
thanks every1