If the filter is rated lower than the fan, the fan strangles on it and air slips past the carbon, so odour gets through. A filter rated higher than the fan is fine. Keep fan, filter and ducting the same diameter, because reducers cost you airflow.
Examples: the High-Flow 600 m³/h, 150 mm suits a 150 mm fan in a 1x1 to 1.5x1.5 tent, and the 1250 m³/h, 250 mm is for big tents or rooms. See all carbon filters.
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