I have found that since my wife like to keep the house at 64-66F that the 50W ceramic heat emitter plus the medium heat pad
on the side is not enough.
I had to use 1/8in thick plexi glass cut to cover 90% of the EXO terra and use a small heat pad stuck to a 1/4 thick ceramic tile
placed under the same side nearest the heat pad and heat emitter I did this so the heat pad is not in direct contact with the bottom glass
witch we all know is a big No-NO. I made sure to place the sensor for the controller near that corner but not in the
path of the ceramic heater so the temp doesn't get above 80F.
I put another temp probe above where the under tank heater is to make sure the temp doesn't get up above 80.
and the controller is doing its job well.
I know that this is a bit much but it works well.
Try Griff's solution first and see how that goes.