I got my gear off a guy second hand 2 years ago, it has been brilliant producing .000 for the last two years. I tested it the other day and it has gone up to .002 which has been fine but I am now looking to replace the Carbon and Sediment filters. Problem is as I got them second hand so i'm not sure where to get the correct filters so any advice will be appreciated that will fit the 2 big blue housing housing. Are filters housing specific or are they interchangeable? Also are filters graded?