The bacteria don't live floating in the water in our tanks, they live in the biofilm which is tightly bound to surfaces. So when we do a water change we remove hardly any bacteria, just the odd few that have become dislodged. We use dechlorinator to remove the chlorine/chloramine from the new water so it can't kill the bacteria already in the tank. And it is why we should clean filter media in water we've taken out of the tank rather than in tap water as the chlorine/chloramine in tap water will kill the bacteria in the filter media.
Chlorine/chloramine does not kill every single bacteria in tap water, a very tiny number do manage to survive. Once we remove the chlorine/chloramine by using a dechlorinator, they can now start to multiply. It's because there are so few of them that it takes so long to grow enough for a tankful of fish.
Bottled bacteria can speed up a cycle. But they don't work if the bottle has been allowed to get too hot or too cold at any point since it left the factory.
Chlorine/chloramine does not kill every single bacteria in tap water, a very tiny number do manage to survive. Once we remove the chlorine/chloramine by using a dechlorinator, they can now start to multiply. It's because there are so few of them that it takes so long to grow enough for a tankful of fish.
Bottled bacteria can speed up a cycle. But they don't work if the bottle has been allowed to get too hot or too cold at any point since it left the factory.