Bio filter media getting dirty and not reducing nitrate

You see any media in your tank including love rock does not have prefiltering and is open to falling prey to crap permeating its porus nature!

That crap being first up, fine particles of silicon from diatoms, fine particles of silica from plankton shells in food and those living in your tank shedding through out their life cycle, all algae sheds through out its life cycle and they contain calcium and more, then from foods there is calcium and silica.
All this begins to fill those wonderful little spaces your nitrate reducing bacteria need to exist in to reduce nitrate!

Then once these have helped seal up spaces any future spaces are filled with organics and more, now your wonderful nitrate reducing areas are manufacturing hydrogen sulphide as obligate anaerobes take over.

All media that is supposed to house bacteria with in will end up this way with out water being prefiltered first up!