here is a nice summation by Eric Borneman in his article in RK, Myyhing the Point:
Through the initial periods, there will be a tank packed with some kinds of bacteria, probably not much of others. Eventually, the massive death slows and stops. Now, what happens to all that biomass of bacteria without a food source? They die. So, another cycle of decomposition begins, and this back and forth process will continue for a while until equilibrium is reached. I say equilibrium, but that is a relative term since reproduction and mortality is a constant process in our tanks, as are "mishaps" and the relative size of the pendulum swing will depend on the reproduction and mortality rates, and biomass of the organisms involved. Still, the new swing of dying bacteria also has antibiotics, toxins, and other substances released when they die. But, the die-off is relatively slow, and is relative to the loss of nutrients, and there is already a huge population present. The result to the aquarist is that they never test positive for significant levels of ammonia. "The water tests fine."