You can add some peat or diatomaceous earth to improve water retention. The sand alone will conduct heat, just more slowly. This won't make much difference because eventually the heat will diffuse away. If you are really getting down to 35F (2C) your problem will be that the heat cables may not produce enough heat to keep your soil at 70F. Of course it depends on how many feet of cable you have in a tub too. That is the unknown experimental variable.
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