Heating?

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Hey guys, I’m setting up my tank before I get a northern blue tongue skink and I’m having some problems with heating. I’ve tried three different bulbs and I just can’t get the temps right. The tank is a 40 gallon breeder. I have a log hide under the lamp for the basking spot and that makes the distance from the bulb about 15 inches. I’m trying to remember exactly what the temps were so I’m kind of guessing at what the temp was around.
The 50 watt halogen bulb only gets the basking spot to 82-85 degrees.
The 75 watt incandescent bulb got it to around 80 degrees.
I think the 100 watt incandescent bulb got it to 85-89 degrees.
Should I try a higher wattage incandescent? With all of the bulbs, the cool side was 69-70 degrees on the cypress mulch. The breeder I talked to recommended I use cypress mulch, but is there a different supbstrate that would hold heat better? I really don’t want to use under tank heating because it honestly scares me to death. What can I do to get the correct temps? I used a temp gun for all of the measurements. I also have a single probe thermometer/hygrometer and on the cool side it’s reading 73 degrees. I have the probe hanging about halfway down the tank, does it need moved? I wasn’t completely sure where to put it because this is my first time keeping something like this. I’m used to animals that will eat or chew anything they can reach so I didnt know how low it could be without a skink bothering it.

[Y33T]

I’d recommend using a tile or rock/rock hide as a basking spot. Wood does not absorb heat well.