Thank you for all your answers.
They look a bit fatter now, the pictures were taken the day after I got them from the pet store and they did look a bit skinny. No need to tell you how well animals are cared for in pet stores, I'm sure you all know.
I'm not sure how cold is too cold though, the cool side of the tank is in the mid 70s and the warm side is at 89ºF according to my thermometer. They were on the cool side of the tanks when I picked them up for the pictures.
Eve