EHEIM - EHFISUBSTRAT replace ??? why ???

I'm running an EHEIM 2224 in a 55 gal freshwater community tank and
have been very happy with my filter (shameless EHEIM plug).

In my 2224 I target to replace the fine white filter pads about every
6 months and the coarser blue filter pads about once a year. I use
the EHEIM pre-filter and find that does a good job of trapping larger
stuff first and I usually rinse that every month to two months. or
when I detect a lessening of flow from the spray bar.

I recently picked up some EHEIM filter supplies and in an instruction
booklet on EHEIM filter material in the package they talk about the
various EHEIM filter media. This is what they quote for EHFISUBSTRAT
(that "sintered glass material").

"When changing the filter medium use approximately 1/3 of the used,
uncleaned material again in order to support the new development of
the bacteria"

I can understand not wanting to replace the entire volume of the
EHFISUBSTRAT and wiping out most of your bacterial colony. My
question is why would you EVER want to replace the material at all? I
didn't think of this material as clogging like the filter pads, but
does it clog? Does it deteriorate? Does it really need to being
periodically refreshed?

And what about the really coarse EHFIMECH (those hollow cermic
cylindrical thingee's). Do those ever need to be replaced?

Thanks in advance.

.... Bill

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