I bet a good soak in bleach would clear out most of the buildup anyways... but I'm with you, I clean my biomedia maybe a few times a year in tank water and don't worry about it otherwise.I would guess that the reason for replacement would be to get the ultimate bio filtering out of each part. It is obvious that some of the tiny holes will get filled and reduce the bacteria space. But with that in mind, I know that there is a ton of excess media in my filter ( if I don't kill them!) so I don't worry about a 10-20% loss of bacteria. I have never replaced my hard stuff in any of my Eheims. I do take it out and pour it into a wire mesh and rinse it in tank water each time.