A twenty gallon tank will only be good for a year, maybe two, depending on how fast the rat snake grows. I have a Texas black rat snake, also leucistic and he's over 5'. He wouldn't fit in a 20 gallon tank comfortably at all.
I would toss out those strip thermometers too..they are not accurate at all. Pick up some digital thermometers with probes and a temp gun. These are much better at measuring ambient (air) temp and surface temperatures.
The infrared light is also useless...as mentioned before. A simple florescent tub light can be used to provide viewing light if you need though they do fine with just ambient light from the room. The UT heater is really all you need to provide the cage with good ambient temps and a hot spot...unless the room gets very cold. If you find you are losing alot of heat, cover the screen top except one small area for ventilation.
You also want to make sure there are no cracks or spaces a baby snake can squeeze out and the lid is secure so it can't be pushed out. Snakes are good at climbing up the side of cages at the corners, they can even wedge themselves between the slip of the tank and the cover, especially when young.
I think snakes do better in one piece cages like shoe and sweater boxes or PVC or mold produced cages than glass tanks. You can find nice clear plastic containers for cheap and its really easy to convert them into attractive cages and they are much easier to keep clean. With a few small holes drilled in the sides or just the plastic lid they hold heat and humidity well and keep snakes in. Just have to make sure the container you buy has a snug tight fitting lid. Some can be pushed up easily on the long sides...though they clamp down tightly at the ends or one long side. Metal clamp style paper clips work great on holding those ends secure too.
-----
PHLdyPayne