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Depending on what micron the second filter is. The main filter will be around 8 micron, brand depending. So if your new filter has finer filtration ability, i.e 8 micron or less, it will need to be after the main filter, to catch what the main filter is unable too, if its like a 15 or 30 micron, before the main filter, to catch the larger particles to assist the main, finer, filter.
Makes sense. Otherwise you would quickly clog the first (finer) filter and have a big restriction. Thanks for the explanation.