Fill gallon containers with hot tap water/ put them inside the tanks or outside small tanks ( up against the glass) and cover the enclosures with blankets. However, as Grif mentioned they need to be changes out frequently

I had to do this - once. I thought I was going to lose my entire collection of frogs .
Never again. It was a nightmare !! I learned my lesson!
A week w/out power and freezing cold !! Like you, I have gas hot water and stove. We were taking turns ,all night, filling gallon containers.

Get a generator if at all possible ! It is wonderful peace of mind !
Lynn