Planted Tank Guru
Join Date: Jan 2010
Location: Contra Costa CA
Just have to think of it this way:
No matter how many tanks you have, how many gallons, how many filters... it does not matter.
If you have F many fish, then you have F x 1,000,000 bacteria.
It does not matter how much bio media you have, or where it is located. If the water from the fish flows through the media, the bacteria will grow there. But only enough to match the waste load of the fish and other organisms in that tank.
But if you had M amount of media, now you have M x 2 media, then each bit of media will only have half as much bacteria, as long as you keep the same number of fish. You are not GAINING any bacteria, just spreading it around.
When you remove the extra media to another tank, you are shorting the established tank of that population of bacteria.
This may not be so important in a thriving planted tank, the plants do a lot of the work. But I would still watch out for ammonia and nitrite spikes when you remove the second filter media.
Just run the 2nd media long enough to get a little bacteria growing, then put it on the new tank and complete the fishless cycle.
Or, take up to 25% of the established media (any- sponge, floss, bio, even month old carbon) from the existing tank, and share that with the new tank to jump start the bacteria growth. I have shared a max of 25% of the established media, even from tanks with no plants, and the donor tanks were fine.
Remember that 25% of the media in the filter does not mean I was taking 25% of the bacteria. There is a lot more bacteria on all the surfaces of the tank- the substrate especially. When I took a bit of the sponge, floss or bio media, I may have been taking 1/8 or less of all the bacteria in the tank.
But that was plenty to jump start the next tank.
I would take that much from several tanks, so the new tank was starting with perhaps 1/4 of a load of bacteria, then I would build up the fish population slowly. (This was before I had heard of the fishless cycle).
I am still doing that concept today. Except that I am adding in the fishless cycle for uninhabited tanks. Gets them ready pretty fast.
Just keep thinking about how much bacteria you have, and where it is.
You can move it around, but you cannot grow more unless you set aside a tank without fish and feed the bacteria with ammonia or fish food.