What kind of bulb is it, mogul end or double-ended? Where is the crack at on the bulb? If you can return it to the company you got it from, I would go that route. If the bulb is cracked along the glass, depending on how bad it is, the gas that is inside the bulb will be released and the bulb will not light properly. It is also dangerous as the bulb could continue to come apart and fall into your tank.