I'm making two assumptions, which if they're wrong make the following remarks way less useful (sorry).
If they're in glass, and heated from underneath you could just insulate the actual cages for the winter. I'm thinking foam panels, or those rolls of insulation at the hardware store (although I freely admit I know nothing about insulation, so if that stuff is dangerous or chemically coated please ignore), basically making a little winter coat for the tank instead of trying to change the 'weather.'
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