I use flexwatt and a Johnson's T-stat. No problems here Also, with my big tub, I cut a square out of the lid and replaced it with the vinyl/plastic coated hardware cloth(it's green, that's all I know). I am using a 60watt CHE and it works awesome.

I will try to take pictures, so everyone can see the sheer genius of the giant snake tub creation, Muahaha! Just kidding though, but I will take pictures.