A fifteen gallon tank for a small Scarlet is a bit excessive.
Indeed, these snakes are quite skittish and oftentimes the only indication that they are still resident in their enclosure is that the food has disappeared.
The 'basking light' you've mentioned is completely unnecessary, as these animals are nocturnal hunters.
Too, I find that the less complicated your set-up, the easier these guys are to maintain.
I have mine in a back-heated rack with a temp. gradient of 80 to 72 F (cooler end dependent upon ambient room temp., obviously).
As a substrate, I use shredded aspen, an inch or two deep.
On one end I place one small, plastic hide box ('Creature Cubbyhole'), and push it down into the aspen allowing only a small gap at the top for the animal to burrow into. On the other end of the enclosure, a small rubbermaid container (with an access hole cut into the side) is provided and filled (loosely) with moss. The moss is kept damp - misted every couple of days.
I provide a water dish at both the front (cooler end), and another smaller one at the back, right against the tub wall. This secondary water source will slowly evaporate (being so close to the heat-source) and provides just the right amount of ambient humidity.
At feeding time, I simply dig a tiny 'nest' into the aspen, put in three or four small, live pinks, and turn out the lights (I always feed my Scarlets overnight).
Unless a particular animal is in shed, the mice will be gone in the morning ...like magic.
One of my WC females was a difficult feeder prior to my having received her and had NEVER taken a pink in captivity. However. utilizing the method described above, she was eating live pinks within just a few weeks of her arrival.
Have patience.
Hope this helps.
Trevor