What percentage of the water in a diesel fuel tank results from humid air entering during engine operation versus entering during storage? Undoubtedly, many factors complicate this question, but can some reasonable estimate be proposed? Let's say a recreational boater in Florida starts out on a day in May with a full tank, ends the day with 3/4 tank, then does not run the engine for a month. It would have been wise to have filled the tank at the end of the day, but that didn't happen. At the end of the month of storage, some water will have entered the tank from water vapor in the air condensing on the tank walls. If some significant percentage of this humid air entered during storage (?), could this moisture have been been blocked by having sealed the tank vent for the month? Or, does a tank need to "breathe" for some reason when the engine is not in use? Further, if a large percentage of the vapor entered during storage (?), and if sealing the vent is effective (?), would a dessicant vent filter provide much additional benefit? How about for a six month layup? Jerry Schupler JSchupler@csi.com