Heat tape is just not getting that warm.

[bcr229]

I've heard of people using flexwatt to heat their homes and that it was first created as a residential heating element. Although all the talk about it not heating anything but the surface seems to contradict that.

Sort of, it was designed to provide under-floor heating in a very small room such as a bathroom. It also requires a lot of insulation under the heating element and a floor with a decent amount of thermal mass, such as ceramic tile. Once the floor warms up, which can take a while, it constantly radiates heat up into the room. A thin plastic tub OTOH doesn't hold heat well, once the source is removed it cools quickly, and tubs aren't well insulated. PVC shelves don't insulate well either.

OP, for a tub you don't need to fill the whole thing with cypress mulch unless you're keeping something like Brazilian Rainbow boa babies that require 90% humidity or higher.

If low humidity is a problem put the cypress in the front few inches of the tub on the cool side and use newspaper for the rest. This is what I do in the winter months as my house humidity runs about 10%, and then I just dampen the cypress every night. It acts like a natural sponge and dries slowly over the next day.