Bacteria will grow to match the ammonia source. No more.
They are not fast growing.
Fastest: Buy a bottle that includes Nitrospira species of bacteria. Make sure you get it from a reputable source, the bacteria are fragile and must not be overheated or frozen. If you use one of these products they say you could fully stock the tank right away. I would prefer to be cautious, just in case you got a bad bottle. Start the fishless cycle, and use the bottled product. If the product is good, the fishless cycle will be complete in about 2-3 days. Then you can fully stock the tank.
Probably faster, but you need the expertise to do it right:
The plant cycle or silent cycle:
Plant the tank so densely that you cannot see the back of the tank. Use at least 50% fast growing nitrogen sinks. Make sure the plants have the right light, CO2, other nutrients. You can lightly to moderately stock the tank depending on how sure you are that the plants are going to be the bio filter.
Works, but gotta do the math:
If you have an existing, healthy tank, then you can split this tank in any of several ways, sharing the bio load (fish, shrimp, snails...) and bacteria, and plants. Make sure the ammonia removers (bacteria and plants) is equal to or greater than the ammonia sources. Adding additional plants to both tanks is good insurance.
One possible way: Put the old, established filter media on a new tank. Split the plants, fish and other livestock 50/50.
The old tank has:
Plants, half the bacteria (in the substrate and on all the surfaces like driftwood, rocks), and half the fish.
The new tank had:
Half the bacteria (it was living in the filter media) and half the fish, and plants.
You can do the math and figure out variations on this. Just make sure you get the ammonia producers and the ammonia removers balanced.
The way you are asking about:
However many fish the system is now supporting, that is how much bacteria is growing in the filters, substrate and so on.
If you add more surface for them to live on, they will colonize it. When you remove this media, you are removing some of the bacteria needed to handle the amount of ammonia the current population of fish is producing.
These bacteria are slow growing. You could get a few going in the new media in a week or two, then move it to the new tank. Then do the fishless cycle to finish cycling this tank.
You will not grow more bacteria than your current fish population if you only seed the media in the current tank. You need to add more ammonia to grow more bacteria.
Here is what I would do:
Take 25% of the established media from the well cycled tank. Replace it with new media. Put the cycled media in a new tank, filter, plants etc. Then do the fishless cycle. By jump starting the new set up with well established media you are getting not just a few bacteria, but the whole complex of microorganisms that have taken several months to get established. By then doing the fishless cycle you are giving the plants and all those microorganisms a couple of weeks to get established and stabilized before adding fish.
Here is the fishless cycle. Takes about 3 weeks if you start with no seeded media. It can go faster if you take some well established filter media from a healthy tank. Do not take so much the tank then shows ammonia or nitrite problems. I have taken up to 25% of the filter media from an established tank, and the remaining bacteria and plants were able to deal with it. More, and you run the risk of an ammonia spike.
Cycle: To grow the beneficial bacteria that remove ammonia and nitrite from the aquarium.
Fish-In Cycle: To expose fish to toxins while using them as the source of ammonia to grow nitrogen cycle bacteria. Exposure to ammonia burns the gills and other soft tissue, stresses the fish and lowers their immunity. Exposure to nitrite makes the blood unable to carry oxygen. Research methemglobinemia for details.
Fishless Cycle: The safe way to grow more bacteria, faster, in an aquarium, pond or riparium.
The method I give here was developed by 2 scientists who wanted to quickly grow enough bacteria to fully stock a tank all at one time, with no plants helping, and overstock it as is common with Rift Lake Cichlid tanks.
1) Set up the tank and all the equipment. You can plant if you want. Include the proper dose of dechlorinator with the water.
Optimum water chemistry:
GH and KH above 3 German degrees of hardness. A lot harder is just fine.
pH above 7, and into the mid 8s is just fine.
Temperature in the upper 70s F (mid 20s C) is good. Higher is OK if the water is well aerated.
A trace of other minerals may help. Usually this comes in with the water, but if you have a pinch of KH2PO4, that may be helpful.
High oxygen level. Make sure the filter and power heads are running well. Plenty of water circulation.
No toxins in the tank. If you washed the tank, or any part of the system with any sort of cleanser, soap, detergent, bleach or anything else make sure it is well rinsed. Do not put your hands in the tank when you are wearing any sort of cosmetics, perfume or hand lotion. No fish medicines of any sort.
A trace of salt (sodium chloride) is OK, but not required.
This method of growing bacteria will work in a marine system, too. The species of bacteria are different.
1a) Optional: Add any source of the bacteria that you are growing to seed the tank. Cycled media from a healthy tank is good. Decor or some gravel from a cycled tank is OK. Live plants or plastic are OK. Bottled bacteria is great, but only if it contains Nitrospira species of bacteria. Read the label and do not waste your money on anything else.
At the time this was written the right species could be found in:
Dr. Tims One and Only
Tetra Safe Start
Microbe Lift Nite Out II
...and perhaps others.
You do not have to jump start the cycle. The right species of bacteria are all around, and will find the tank pretty fast.
2) Add ammonia until the test reads 5 ppm. This ammonia is the cheapest you can find. No surfactants, no perfumes. Read the fine print. This is often found at discount stores like Dollar Tree, or hardware stores like Ace. You could also use a dead shrimp form the grocery store, or fish food. Protein breaks down to become ammonia. You do not have good control over the ammonia level, though.
3) Test daily. For the first few days not much will happen, but the bacteria that remove ammonia are getting started. Finally the ammonia starts to drop. Add a little more, once a day, to test 5 ppm.
4) Test for nitrite. A day or so after the ammonia starts to drop the nitrite will show up. When it does allow the ammonia to drop to 3 ppm.
5) Test daily. Add ammonia to 3 ppm once a day. If the nitrite or ammonia go to 5 ppm do a water change to get these lower. The ammonia removing species and the nitrite removing species (Nitrospira) do not do well when the ammonia or nitrite are over 5 ppm.
6) When the ammonia and nitrite both hit zero 24 hours after you have added the ammonia the cycle is done. You can challenge the bacteria by adding a bit more than 3 ppm ammonia, and it should be able to handle that, too, within 24 hours.
7) Now test the nitrate. Probably sky high!
Do as big a water change as needed to lower the nitrate until it is safe for fish. Certainly well under 20, and a lot lower is better. This may call for more than one water change, and up to 100% water change is not a problem. Remember the dechlor!
If you will be stocking right away (within 24 hours) no need to add more ammonia. If stocking will be delayed keep feeding the bacteria by adding ammonia to 3 ppm once a day. You will need to do another water change right before adding the fish.
A) You can run a fishless cycle in a bucket to grow bacteria on almost any filter media like bio balls, sponges, ceramic bio noodles, lava rock or Matala mats. Simply set up any sort of water circulation such as a fountain pump or air bubbler and add the media to the bucket. Follow the directions for the fishless cycle. When the cycle is done add the media to the filter. I have run a canister filter in a bucket and done the fishless cycle.
B) The nitrogen cycle bacteria will live under a wide range of conditions and bounce back from minor set backs. By following the set up suggestions in part 1) you are setting up optimum conditions for fastest reproduction and growth.
GH and KH can be as low as 1 degree, but watch it! These bacteria use the carbon in carbonates, and if it is all used up (KH = 0) the bacteria may die off.
pH as low as 6.5 is OK, but by 6.0 the bacteria are not going to be doing very well. They are still there, and will recover pretty well when conditions get better.
Temperature almost to freezing is OK, but they must not freeze, and they are not very active at all. They do survive in a pond, but they are slow to warm up and get going in the spring. This is where you might need to grow some in a bucket in a warm place and supplement the pond population. Too warm is not good, either. Topical or room temperature tank temperatures are best. (68 to 85*F or 20 to 28*C)
Moderate oxygen can be tolerated for a while. However, to remove lots of ammonia and nitrite these bacteria must have oxygen. They turn one into the other by adding oxygen. If you must stop running the filter for an hour or so, no problem. If longer, remove the media and keep it where it will get more oxygen.
Once the bacteria are established they can tolerate some fish medicines. This is because they live in a complex film called Bio film on all the surfaces in the filter and the tank. Medicines do not enter the bio film well.
These bacteria do not need to live under water. They do just fine in a humid location. They live in healthy garden soil, as well as wet locations.
C) Planted tanks may not tolerate 3 ppm or 5 ppm ammonia. It is possible to cycle the tank at lower levels of ammonia so the plants do not get ammonia burn. Add ammonia to only 1 ppm, but test twice a day, and add ammonia as needed to keep it at 1 ppm. The plants are also part of the bio filter, and you may be able to add the fish sooner, if the plants are thriving.