I think the pH issue is really pretty simple to deal with if you think of the aquarium as a closed biological system. This means that unless you perform routine wcs, there is no way to prevent the accumulation of acids and there is no opportunity to introduce anything to buffer these accumulating acids. First, where do the acids come from?
1. The fish and aerobic bacteria expel CO2 and this combines with H2O to form carbonic acid. The carbonic acid then dissociates to relase H+ and this causes the pH to drop.
2. When the fish defecate and urinate and when the bacteria in the biofilter die, they release various organic acids. Some of these molecules dissociate to form the corresponding conjugate base and release a H+ in the process. The more H+ released, the lower the pH.
Let's say that I have an aquarium containing 100 gallons. This 100 gallons of municipal water has a certain finite amount of carbonate and bicarbonate (KH) which will buffer and counteract these accumulating acids (H+). However, because this is a closed system, eventually the KH will be exhausted and you will then see the pH drop, since the accumulating H+ is no longer being absorbed (buffered). Regardless of the remaining buffering capacity, life goes on inside that 100 gallons and H+ will continue to increase. So, what do you do to remedy this?
Really, the only practical approach is to a. remove the accumulating acids and b. reestablish the KH by adding carbonate/bicarbonate. Both of these actions are accomplished by performing regular wcs. You can further firm-up this buffering system by adding sodium bicarbonate (baking soda) to your aquarium water. Others appear to prefer adding something like coral or dolomite to their filters...both of which, theoretically, will contribute carbonate to the water.
As far as the role of nitrate in lowering the pH....I doubt it. For this to occur, the nitrobacter would have to be releasing HNO3 (nitric acid) and this does not occur.
