Clogging of ceramic/fine porus bio media.

Firstly a biology fact, some bacteria have the ability to form biofilms!
They go through the processes of growth,repropduce, die off and all the time these are attached to any type of media.

This ubundance of dead tissue that does not completely disappaer, asimilates with the rest of the detritus with in our waters that are in particle size, quite often to small for a filter sock to filter out of the waters and they by pass this barrier!
You combine these with the multitudes or calcium carbinate particles and its precipitation, silica and metals potentialy becoming part of hese bio films and your fine porus media is most certainly becoming clogged as oxygen and waste products diffuse into the cracks, creases, and pores of the bio media!
The hundred square meters of surface area over time, will become just several square meters.

Bio films.
http://www.ncbi.nlm.nih.gov/pubmed/11018124

Particles held by these bio films.
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC93159/

Clogging from precipitation.
http://www.reefkeeping.com/issues/2005-07/rhf/