Planted Tank Guru
Join Date: Jan 2010
Location: Contra Costa CA
Beneficial bacteria will grow wherever the conditions are right.
Some, like nitrifying bacteria, demand very high oxygen levels and would not grow on this if it were covered in soil. They will grow on almost anything that is in a high flow area, such as a filter. They will grow in the substrate only when you are forcing water through it, like the old style undergravel filter. Otherwise they will just grow on the top layer of substrate, perhaps 1-2 rocks deep, where the maximum oxygen is.