Let's clear up some things.
In an established tank one has little idea of how much bacteria is where. It can be in filter media, it can be in the uppermost layer of substrate (under an inch deep) it can be on the underside or any decor as well. The bacteria is somewhat photophobic so the places it will not be is where there is decent light.
Yes, one can use something from a cycled tank to seed a new tank. And yes, in many tanks the biggest concentration of bacteria may be in one's filter media, but it also may not be the case. There are a number of variables which will determine where the bacteria are in the greatest numbers.
What we do know it that in an established that there will be as much bacteria as the ammonia levels require to make things safe. Add more fish and there will be more ammonia and this sets the nitrifying microorganisms to reproducing to handle it. Conversely remove some of the fish and the bacteria slow reproducing and the result is there will be lees of them.
What this means is when we move cycled media out of a tank we are removing some amount of the bacteria needed by that tank to keep it safe. Again, we have no real idea how much bacteria we may be either removing or leaving behind. Remove too much and there can be an ammonia and nitrite spike in the tank from which they were taken. Bear in mind there is no reason for us to assume that when we have two identical sponge filters in a tank that they both hold the same amount of bacteria. All we really can know for sure it that a tankis cycled or it is not. The one exception to this would ba a bare bottom tank with no decor which has a single filter. And even then we cannot assume that the bacteria in that filters media have an equally distributed number of bacteria unless it holds a single sponge.
On the other hand, we have no idea how much bacteria we have put into the new tank. What we do know is it is likely not enough for the tank to be stocked fully nor what partial loading may be OK. So how should we be doing things to make both tanks safe for the fish we have or will out into them?
So it is important to test the older established tank from which we have taken some amount of bacteria to make sure there is no spike of either ammonia and/or nitrite. The threat from this is usually minimal as long as we do not remove too much bacteria.
Next, how many fish can we put safely into the new tank? There is little way to know until we actually put them in and test. However, there is a better way to do things. But first, it is important to consider whether either of the two tanks contains live plants. These use ammonia and short circuit the need for some amount of cycling. Not only do the plants use ammonia, but they also host the desired bacteria on them.
The easiest way to deal with the new tank is as if we were doing a fishless cycle with plants. In that case, and in my opinion, the best course of action would be the following. Instead of moving the media, rinse it out in the new tank. Do not worry about the discoloration of the water when doing this as it contains the bacteria you want. They will end up in both the filter and the substrate. This sould also insure enough bacteria is left in the established tank because it is impossible to rinse it all out this way.
Next, depending on the presence of plants or not, add ammonia as follows: for a planted tank add a maximum of 2 ppm of ammonia while for an unplanted tank this can be 3 ppm. Then test daily for both ammonia and nitrite. Do not add more ammonia until the level comes down to .25 ppm of lower. Do not add more ammonia if the nitrite is above .5 ppm. When you can add another dose of ammonia, wait 24 hours and test. The tank is fully cycled when you can test 0/0 for both ammonia and nitrite in 24 hours. At that time you can add up to a full stocking of fish to the tank.
However, if you know you plan to stock the tank gradually from the outset, you can cycle using a lower concentration of ammonia. What is important is that the more fish you want to add right away, the more ammonia you should use up to the above mentioned limits.
The above methodology will prove to be he safest for all the fish involved in both tanks.
Some years ago I found that I needed to be able to cycle anywhere from 4 to 8 tanks for temporary summer use of for when I would be selling in the vendor room of weekend fish events. So instead of trying to cycle 4 to 8 tanks individually, I began setting up a bio-farm for cycling filters rather than tanks. All the needed foam filters were cycled in a bio-farm tank that only held about 30+ gals. but which would eventually be placed in 200+ gallons in total across 8 tanks.
It normally takes me between 10 days an 2 weeks to get all the filters cycled. The longer time is usually the result of making a small mistake on my part. Things like letting the KH drop too low or allowing nitrate to build up too high happened because I failed to test or to do a massive water change on time. To get this sort of speed I also rinse out a few of my foams from established tanks and add a small bottle of Dr. Tim's One and Only. A few extra days of ammonium chloride additions are way cheaper than enough bacteria to cycle all the filters.
As always the above is not the only way to do things, it is just how I do them to avoid issues and to get the fastest results I can. In my case all of the tanks will be stocked to capacity right away. There is no gradually adding fish.
edited for typos and spellingaggitation
