Thanks Diana! I was hoping the combo of the 108 gallons/12 tetras and the added media from the tetra's old tank & rocks from two others would do just that - keep the ammonia spike minor.
The QT tank the tetras came from was a bare bottom tank - what's in the stocking is a large (for a 10g tank) hunk of polyester quilt batting that I had in the HOB filter to polish the water and harbor beneficial bacteria for just this use. I also have the coarse sponge-like filter insert that goes with the HOB - not to be confused with the carbon insert, something separate. The QT tank is empty now and I don't have any plans for adding new fish to the 125 for quite a while (it may take me months to finally decide if I want to buy congos or put my existing Buenos Aires in there...), so I feel fine using these things in the 125. The driftwood is new and one piece is already getting it's bacterial bloom - something I love to see - lol.
Today is water change day for the other tanks, so I'll take some readings on the 125...I guess I need to start daily tests on it.