12-22-2011, 12:26 AM
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In those situations obviously the bacteria are not in the filter since there isn't one. In those setups the bacteria will live throughout the tank.
Bacteria need certain things, oxygen and food are two of the most important. These are highest in the filters because of the flow. Bacteria are not going to simply grow where they CAN, they are going to grow where it is best. Think about it in terms of humans. We don't simply spread out evenly and live where we can, we concentrate ourselves around what we need, frequently water. Of course this doesn't mean that there are not people living high up on mountains or in remote cold areas, it just means that there are very few people in those places.
I have seen many situations demonstrate this very well. On multiple occasions I have moved the fish and filters from one setup to another at the same time. No water, decor, or anything else was moved, just the fish and the filters. The new setups did not experience any mini- or re-cycle at all because effectively all the bacteria were in the filters.
Obviously this isn't 100% in every tank. Some tanks barely have enough filtration or biomedia, so there will be more bacteria throughout the tank in setups like that.