Can substrate be too cold?

[ashleylovesyou]

My apartment has been especially frigid these last few days, so I got a 3rd UTH for my 20gal tank. Thankfully, that put the temperature back into the 70s, but I have no idea what temp the substrate is. When I put my hand on the outside of the tank down by the substrate, it feels pretty cold. I know UTHs are supposed to be placed on the sides of the tank, above the substrate (which is how mine are set up), so how am I supposed to keep the substrate warm? 3/4 of my crabs are now molting and I'm worried it's too cold for them.

[amandapri82]

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[wodesorel]

Crabs dig down in order to cool off, so normal room temp (65+) is okay for when they bury. Ideally it's warmer towards the surface so they can find a comfortable location.

This is one of the main reasons why I have my UTHs cover the entire back, so there is some warmed substrate that they can choose to molt in. Room temp where my tank is located is normally 50-60 degrees which I feel makes the substrate farthest away from the heater too cold.

One thing to be cautious of is majorly changing the temperature once they are down molting, as they are stuck in place until they finish and could end up too hot or cold.