Dissimilation

Dissimilation is used in this context with anaerobic heterotroph bacteria in the aquarium hobby.

That is heterotroph bacteria live in low (DO <2.0 ppm) to no oxygen enviroment and will actively convert nitrogen back into nitrate and nitrite. This is something we generally don't want.

These species of bacteria live in the substrate or other areas where fresh water can not reach them.

If you have small particles of substrate, like sand for example, then this can compact, especially if the layer is more than $$ thick and prevent fresh oxygenated water from penetrating it.

Some aquarists will stir up their sand layers with a stick ever week to ensure this type of bacteria will not proliferate.