Do I ever need to replace the substrate in my ehiem filter, those 'little rock like things' that make up the bio filter and the 'little white hollow things' that go before the first filter pad? I've been using the same ones for the last 2 years and never had any problems with my parameters. I just rinse them out with aquarium water whenever I go in there. They do look a little stained however. i'm sure the manufacture recomends replacing them, but is that just to boost sales?