krusty - TLC's StartSmart isn't anything like the septic tank products being discussed in this thread - StartSmart is for cycling new tanks, and according to the manufacturer contains 100% nitrifying bacteria.
As far as fish food that contain probiotics, there are a few out there, and I'm sure there will be more to follow, those products contain the exact same types & strains of bacteria as the septic tank products being discussed here. The main difference being is that the septic tank products can be purchased for a teeny tiny fraction of the cost of ANY type of probiotic found at your LFS. One doesn't have to feed their fish soybean meal laced food in order to add probiotic bacteria to their system.
Shadow - in an established tank what you suggested will never happen. While heterotrophic bacteria will die off when the organic load diminishes, in an established aquarium there will always be a food source present for these bacteria as there will always be organics present. The work-around to those waste eating bacteria that die off is simply to add more product on a weekly basis to keep the numbers up. Nitrifying bacteria (the other "good" bacteria) consume ammonia, so they too will always be present and will not die off from the addition of waste consuming bacteria such as what's found in septic products.
