Anytime you buy used equipment you need to sterilize it, the way I do it is to ad a 80% white vinegar 20% extremely hot water(not boiling but around 180-190 degrees) blend and use that. To clean a filter you can simply fill the filter up with the mixture and a bucket, and then put the intake and outflow into the bucket and run it for a while, I usually let it run for 30 minutes to an hour but I like to overdo things, probably 10-15 minutes would work.