The glass becomes fogged when the inside, more humid tank is warmer than the outside temperatures, so the water condenses on the cooler glass--(dew point temperature.) There are some technical ways to prevent this such as installing a ventilation/fan system across the inside front of the tank with vent holes or section of screen across the front of the top, or a partial screen across the front of the tank, etc. You could probably also just blow warm air on the glass from the outside to warm the glass.

I usually just wipe the condensation off with a paper towel, so have no personal experience with doing the ventilation thing. Otis7 or someone else may be able to provide more details about how to set this up.
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Patty
Pahsimeroi, Idaho

Dendrobates: auratus blue, auratus Ancon Hill, tinctorius azureus, leucomelas. Phyllobates: vittatus, terribilis, lugubris. Epipedobates: anthonyi tricolor pasaje. Ranitomeya fantastica, imitator, reticulata. Adelphobates castaneoticus, galactonotus. Oophagia pumilio Bastimentos. (updated systematic nomenclature)

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