The nitrogen cycle is often in the spot light of discussion, while the carbon cycle is usually ignored. The bacteria we grow in biological media require a nitrogen source which is supplies as ammonia from the gills of our fish, as well as a carbon source provided from carbonates in the water in order to function and reproduce. As the bacteria reproduce and die, they generate a lot of cellular debris when combined with solid waste from the fish, accumulates in filter media, substrate and surfaces.
I use lee's economy corner filters with 1/4 inch lava rock gravel mixed with Caribsea crushed coral, which is actually mined Aragonite not crushed corals. Crushed coral is poor buffering media as they can require pH as low as 7.2 to dissolve. Aragonite on the thee hand starts dissolving below 8.2 pH.
As the aragonite dissolves, it buffers the carbonate hardness by releasing carbonates, increasing the pH as well as the general hardness as it releases calcium. With the use of the aragonite "crushed coral" and or a little aragonite sand in the tank and your buffering needs can easily be accomplished.
The fine grain nature of the media with a slow trickle of air traps fine carbon sediment in the box, which is much easier to clean than a sponge filter. A couple shakes and rinses and your clean and back into service.
This set up of course provides considerable biological filtration as well.
