I think some strains when introduced take advantage of the available food which has accumulated over time, and grow and eat at an amazing rate. But I think they eat themselves out of house and home so to speak (unless you supplement their food supply) and they all die. So when the food (organics, nitrogen, what ever) builds back up, you need to put some bacteria in and again clean things up.
Also, like Cary says, there are so many other factors which affect specific bacteria's vigor and effectiveness.
We used bacteria to break down petroleum and chlorinated solvent contamination in soil and ground water. The bacteria are often there but they didn't thrive, so they weren't working at full potential. We would add food or air to the ground water or soil, and the bacteria would take off and work overtime. We would sample to see what bacteria were present before we added supplement obviously.
I've been impressed with that which I have used in my pond. Where I once had sludge on the bottom of the pond, I can now see a sand bottom. I've got one more bag and when the water temp gets to 60, I will add it to the pond. But I've lost my net link where I bought the stuff and need to do some surfing or look at past charges to the credit card to find the source (sorry I can't endorse that specific product at this time).
