The Tilapia will "farm" detritus by forcing oxygenated water onto and under detritus to promote aerobic bacterial growth. The tilapia get nutrition from the bacteria growing and decomposing the detritus.

Turtlemtn, "Muck" builds up as you "guessed"...from a heavy influx that existing bacterial colonies can not consume. The muck comes in, like a fall leaf dump, and cover the bottom and existing aerobic bacteria. Water under the new leaf quickly becomes devoid of oxygen (anoxic) as the aerobic bacteria consumes the oxygen....once the O2 is gone, your "beneficial bacteria" die. Since water can't circulate under the leaf litter, anaerobic bacteria form. Anaerobic bacteria does consume detritus, but at a rate about 1000 times or more slower than aerobic bacteria does. Anaerobic bacteria also produces hydrogen sulfide gasses that can be poisonous to the fish and aerobic bacteria and is noticed by the "sewage" smells produced.