Hi all,
Like Clive says, it doesn't matter as much as you might imagine. This is really because the micro-organisms that decompose organic products are a complex assemblage of species that have evolved over billions of years.
We used to have one of these in the lab. <
Winogradsky column - Wikipedia, the free encyclopedia>.
If a scarce product like fixed nitrogen (NH3, NO2 etc) is available in any environment, organisms will have evolved to exploit it. Low pH, high levels of CO2 and low levels of oxygen are very common in environments where decomposition is occurring, principally because the bacteria are utilising oxygen (O2) and producing CO2.
The difference is in the end product produced (e.g. CO2 if aerobic decomposition of organic matter is occurring or H2S if anaerobic), and in the efficiency of the process. In our terms biological filtration is all about oxygen, as long as we have enough oxygen everything else is much less important.
A lot of the problems people attribute to low pH etc are really to do with low levels of dissolved oxygen (it is the O- in OH- that is the base) leading to a failure in biological filtration.
Fish bits
The practical ramifications of this were why I wrote: "
Aeration and dissolved oxygen in the aquarium": <
plecoplanet: Aeration and dissolved oxygen in the aquarium>.
Some might also be interested in this page on fish physiology and the Bohr-Root effect: <
Fish Respiration>.
cheers Darrel