Fishhead, Mario and others, great post on bacteria.
I attended a 3-day course for Bacta-Pur. Beautiful land there Mario. The strains are found in the ponds but at what amount and depth is depending on water quality. There are also different strains for different objectives as stated earlier, saltwater vs. freshwater, pelagic vs. bottom sluge digesters.
Obviously you have anerobic and aerobic forms and falcutative too. By providing the correct habitat for the form you desire (aerobic being the most effective) you will achieve the results you desire.
By adding the bacteria to the pond you are ensuring that the strains needed are introduced to the pond. Yes they will grow if the habitat is there. If you find that you are not achieving the desired results, Alkalinity and pH could be your damaging factor.
Keep in mind that efficient water purification is a funtion of balanced communities of bacteria, not single strains. Balanced strains are often lacking in nature and by enoculating (sp?) with a quality bacteria product that guarantees types of strains and counts of strains/g as Mario stated you are balancing out the bacteria communities in your pond.
Most of Bacta Pur products contain 12 strains to nitrify, denitrify and they even have the ability to reduce nitrate accumulation.
In a nutshell, if you provide the habitat and at least enoculate once a season, you are giving your pond that "flu-shot" for the season.
