process
Mechanical filtration
Trapping detritus makes water clearer and concentrates waste where it can be taken out. It becomes counterproductive if the trapped material is left to decompose inside the filter, which simply returns the nutrients in dissolved form.
In one sentence
Mechanical filtration traps suspended particles so they can be removed from the system. It does not remove anything dissolved.
Getting the sequence right
Water should meet coarse material first and fine material last. Fine floss placed first clogs within days, flow falls, and the biological media downstream loses oxygen. Coarse-to-fine also makes cleaning far less frequent.