Each tank requires a ventilation (or: breather) opening.
Unfortunately it's not just fresh air that escapes through the breather of your fuel tank. In order to stop the diesel fumes from entering your cockpit a Craftsman Marine smell absorber can be fitted to the breather line, before it leaves the boat. The filters are provided with an active carbon filter element. These filter elements are separately available and it is recommended to replace them before the start of the new boating season. Old elements can be disposed of in the regular garbage.