A gradient measured where the skink lives
A blue tongue skink is a heavy-bodied ground dweller who experiences his enclosure at floor level, so his gradient is a floor gradient, not an air one. At the warm end he needs a basking surface of 95–105°F — northern blue tongues sit happily at the top of that range — with the surrounding warm-end air in the low 90s. The far end should fall to 75–80°F. Between the two, give him deep, loose substrate he can bulldoze into: a skink thermoregulates partly by burrowing, slipping down into cooler, damper layers when he has had enough heat and surfacing to bask when he needs to warm up. A flat, bare floor forces him to choose between two air temperatures; a deep floor gives him a whole staircase of them.
Setting up the warm and cool ends
A basking lamp over a flat tile or stone at one end makes the hot spot; tiles hold heat into the evening, which suits an animal that likes to sprawl on warmth after the lights dim. Run the lamp on a thermostat, and measure the tile surface with an infrared temperature gun — the air thermometer on the wall will not tell you what his belly is feeling. Put a solid hide at each end so he can feel concealed at any temperature, plus a water bowl big enough to soak in at the cooler side. Tropical subspecies such as Indonesians also want noticeably higher humidity than the Australian northerns, so their cool end doubles as the damp end, with misting or a humid hide; a skink who cannot shed heat or find moisture starts to look dull and sheds in stuck patches. At night, a drop into the low 70s is natural and needs no correction in most homes.
Reading the signs of a wrong gradient
A skink with a working gradient has a daily rhythm: out basking in the morning, a slow patrol, then long buried hours with just the top of his head or nothing at all showing. Worry about the setup when he is pressed flat against the cool-end glass all day, when he soaks constantly in his water bowl, or when he stops emerging to feed — all classic signs the warm end is running too hot or the cool end is not cool. The opposite picture, a skink who basks from lights-on to lights-off and still feels sluggish and eats without enthusiasm, points to a hot spot that never quite reaches temperature. Blue tongues are famously food-motivated, so a skink turning away from favourite foods is giving you information, not being fussy.
When to bring a vet into it
Take gradient problems seriously when they come with physical signs: wheezing, bubbling at the nostrils, or open-mouth breathing in a skink kept too cool and damp suggests a respiratory problem that needs a reptile vet promptly, and a skink who has lost weight despite eating, or whose shed stays stuck around the toes and tail tip over repeated sheds, deserves a visit too. Bring your floor-level readings with you — basking surface, cool-end substrate temperature, and night minimum — because they let the vet separate a husbandry fix from a medical one. When the numbers still will not settle mid-adjustment, put a follow-up question to KinsLe's Kimmy; she answers using the KinsLe Answers library.