The need for fresh air and all other climatic factors vary significantly in accordance with their origin.
Nowadays, the terrarium is usually ventilated through two air screens mounted on different sides which prevent stagnant air from accumulating. Fitting a ventilation grid under the front panes has the advantage of keeping the view into the terrarium unobstructed. The air in a terrarium heats up from the heating mats or cable on the floor, the lighting and the radiators and then rises subsequently. Some of the warm air escapes through the ventilation grids, usually in the top of the terrarium, allowing fresh air to flow in through the grid under the front panes. The air circulation helps keep the panels and furnishings dry.
If there is no ventilation in the lower third of a humid terrarium, stuffy congested air saturated with moisture forms quickly, causing the front panes to fog. This is why aquariums are only suited for keeping animals from dry regions (e.g. leopard geckos) and not for setting up a rain forest terrarium unless ventilation slits are placed near the floor subsequently. If diurnal reptiles such as tortoises are kept in aquariums, large ventilation surfaces in the cover must stay open as well. This leads to a high loss of heat and moisture, which is why aquariums can only be recommended without reservation for very few animals. In a terrarium, the optimum moisture can be achieved by increasing and decreasing the size of the ventilation openings. The heat does not escape as quickly and in as large amounts as from an open aquarium.