I use a few UTH's under the tank, and the plaster conducts heat beautifully (I use plaster disks to conduct heat evenly in ovens when I bake clays - I'm a sculptor) Plaster is known for it's ability to conduct and diffuse heat well, so it's actually better than just sticking the UTH on a glass bottomed tank.

I also embedded a couple of heat rocks in the plaster. These rocks are tested and on a dimmer switch. There is a thick covering of plaster over them to diffuse the heat, and I only plan to use them in the winter if my house gets cool. The under tank heaters are doing a great job alone right now.

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