You can use the reptile mats applied to the background on the outside, or if you have a metal mesh stand, underneath, but I've had some bad experiences with placing them under a tank that is on top of a wood or counter top. They are not thermostatically well controlled and may overheat and scortch the stand. They are also relatively inefficient on the outside. There used to be a very good nursery mat that was designed to go under damp soil for seedling starting, but they were very expensive, and I haven't been able to find them again since I used them in a greenhouse about 25 years ago.
What are your room temperatures? Your lighting should add heat to the tanks that will be taken up by the soil and water, and gradually dissipate through a cool night, keeping the vivarium somewhat warmer than the ambient room temperature. Your vivarium will always be somewhat warmer than the room. Many darts can take cooler night temperatures than the recommended 65 degrees and still do just fine as low as 55. It may take them some time to wake up and smell the coffee in the morning, but it won't kill them or even affect them in the long run. I live in SE Idaho where it's really cold in winter and sometimes during a power outage, I have to supplement with a wood stove. (A heat mat or lights are useless in these circumstances, of course.) Ironically, I have the most problems with keeping cooler tank temperatures in summer, although the outdoor temperatures seldom get above 80.
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Patty
Pahsimeroi, Idaho
4 D. auratus blue
5 D. galactonotus pumpkin orange splash back
5 D. imitator
6 D. leucomelas
4 D. pumilio Bastimentos
4 D. fantasticus
4 P. terribilis
4 D. reticulatus
4 D. castaneoticus
2 D. azureus
4 P vittatus