Well, I am using a Herpstat on my flexwatt, and yes it keeps my hot spot at the right temp. But that doesn't keep the light from raising the ambient temp. From what you said about checking the fixture for size and connection type to find a suitable bulb - I'm not sure you read my post right. The bulb I'm using fits the fixture, it would have to in order to be on and heating up the tank. Finding bulbs of the right size and shape to fit the fixture is not a problem, I'm asking what bulbs people have used that didn't warm up and affect their temps.