I have a (crappy) halogen reptile bulb and a UVB bulb in one fixture sold for reptiles... That was recommended by the pet store... They were using them on their tanks. When I started using it, they stopped basking.

so, I added back the Flukers heat bulb that I had originally along with the uvb fixture and they are back to being happy baskers! I adjust the temp by moving the flukers bulb father away and take the air temp several times over the course of an hour until I'm sure the temp won't fluctuate. I have a thermometer that is a temporal one (used on foreheads but can take air temp as well).