Question about reading temps

[Zincubus]

What is the lowest temp that is safe/healthy that I can have on my cool side??

What I also think is causing this particular tank to heat up more is the fact that it is stacked on top of another T8 with an RHP and getting residual heat from that.

That certainly happens with stacked vivs . At least you can compensate and make adjustments . I once kept a corn snake for a short while in a top viv of a stack for a while and unplugged the ceramic as the temps were perfect and the heat was coming up from the viv below .