There is no way we can actually know where the bacteria actually are in our tanks. We can only make assumptions. What we can know is whether we have enough to do the job or we do not. For example there will be a difference in the location of most of the bacteria if one is running a bare bottom tank. It will change with the addition of live plants or the type of media we use in filters.
That said we can know a few things. The bacteria live in a biofilm that attaches them to a hard surface. Most of the bacteria do not move. But some small % can when it is needed. So where the will live in the greatest number is where they can get the most of what they need. This basically means ammonia/nitrite, oxygen, inorganic carbon and some iron. The other thing is they are photophobic. The darker it is, the better.
Next, we do not tend to see out bacteria as millions of individuals, we thing of them as a unified entity. Every day some number of individuals die and others are born via division. In a stable tank the births and deaths tend to equal out and the colony remains a stable size. What will change the size of the colony is ammonia. If we add more fish to a tank, more ammonia will be produced. The bacteria can sense there is more ammonia than they need and the reproduction rate will increase which results in the size of the colony increasing.
One the other hand of you remove a number of the fish the amount of ammonia being produced will decline. Again, the bacteria sense this and the rate of reproduction slows. More individuals will die than are being "born." So what happens in our tanks in terms of the bacteria is the colony will always size to the ammonia load.
When we put another filter on an established tank it does nothing to increase the number of bacteria. But, most filters are a good place for bacteria to live. Some or those motile individuals will find their way into the media in the new filter. They will multiply there while in other places in the tank which get less of wheat the bacteria need, they will die back faster. The net result is it appears as if the bacteria have moved, in reality they hand not. The amount of working bacteria in the tank will remain the same as no more ammonia is being created.
So, depending on the nature of an existing tank, some bacteria will "move" into the new filter and out pf other places. Since we cannot see them, when we remove one filter to another tank. we have no idea how many are being moved and how many are left behind. This makes it imperative that we test both tanks for ammonia for a bit after moving the filter to a new tank.
I have developed my own way of seeding filters and moving things around. It is not the filter that does the filtration, it is the bacteria. They live in the media, in the substrate, on the darker areas of rock and even wood. I also know that under optimal conditions the ammonia ones need about 8 hours to double in number and the nitrite ones take a few hours more. What this means to me is that if I only move 1/4 of the bacteria that the old tank will handle the fish load completely withing about 3 hours. I also know that the spikes will be smaller and will decrease fairly fast.
So, I prefer either to move some media to a new filter or, because I have enough tanks, to rinse out the media from a few of them into the new tank. I leave the lights off and run the filter. That bacteria will get into the media and the substrate. But, there may not be enough bacteria to add if one doesn't have several tanks.
So, I have two option. I can start to stock slowly or I can act to increase the number of bacteria. I do this by dosing ammonia like i would in a fishless cycle. But, this is different than a normal fishless cycle because we already have 25% of both type living in a biofilm already and quickly attached to surfaces. What we do know is that whatever amount of ammonia can be processed into nitrite by the bacteria we moved over, there are sufficient bacteria to process that nitrite. So of we increase the ammonia level in the tank, both types will be reproduce at the same time. The sequence of ammonia first and then nitrite next no longer exists. And this speeds up the process a great deal.
Where it might take 5- 6 weeks to cycle the same tank from scratch, a 1/3the jump start of both bacteria means inside of 2 days we could have the tank fully cycled. But, since we cannot really know how much bacteria we started with, we can not know how long it will take them to double. But our test kits will tell us when they have done so. In most cases 2 ppm of ammonia is more than enough to ready the new tank unless the pH is over 8.0. The makes the ammonia more toxic and the higher from there it gets the more deadly the same level of ammonia will become. If you have higher pH water, use 3 ppm.
What you want to see either way is that you can add that amount of ammonia to the new tank and 24 hours later you can test 0/0 for ammonia and nitrite. That is the only way to learn for sure how safe from ammonia the tank may be.
One caveat if you do live plants, some of them are not able to tolerate elevated ammonia. However, they do consume it as ammonium. So, in such a tank I would not exceed 2 ppm of ammonia. And. for a few plant species which most of us do not keep, even 2 ppm is too much. I would hope if you are keeping such plants you know most of this already.
The point here is we can never know the quantity involved in a tank or where in the tank it is when it comes to the bacteria. Our test kits are all we have to get an answer. If there is enough bacteria present, the readings are 0. if there is not enough, we will get readings above 0. The level of such readings and a bit more info (pH and temp.). will show us what we should do.
