Many organisms. Different pods, worms, some stars. Some just break down to bacteria eaten by other inverts. A diverse and healthy tank should have all the necessary animals to eat all food sources. If you have plenty of live rock you should do ok. If you see a build up of detritus on your rocks (read flakey, dusty mulm) use a powerhead or turkey baster to blow it into solution so your mechanical filtration can export it.