Hi,
That position on substrate depth originated I believe from non planted tanks where the risk of anaerobic bacteria might be higher. As the depth of the substrate increases the oxygen levels at depth decreases. "Aerobic" nitrifying bacteria (the germs that convert ammonia to NO2 and then to NO3) require oxygen to perform this transformation - it should be obvious that in order to create the "O2" and O3" portions of those NO2 an NO3 molecules they would need a source of "O". When there is low oxygen levels these particular germ species may become extinct and are replaced by other germs which do not require oxygen. These other germs are referred to as "anaerobic" bacteria and they convert the ammonia to other chemicals, sometimes noxious, depending on which other chemicals are available at these depths. Often, sulphur is used and the result is Hydrogen Sulphide (H2S) which itself is toxic and even flammable. This is the rotten egg smell which sometimes forms in the substrate.
Plants in tanks with good flow, CO2 and nutrients generate root structures that penetrate and colonize the substrate. During photosynthesis oxygen is generated which the plant uses for cell metabolism. Plants transfer oxygen to their root structures which then escapes into the substrate providing the aerobic bacteria with the required O2. As a result the substrate depth in a planted tank is not really a limiting factor assuming the plants are healthy.
Cheers,
