http://www.thekrib.com/Chemistry/ammonia-discussions.html
QUOTE
Hi Susan etal. I just wanted to mention that cycling a tank at a
pH below 7 is not necessarily safer: while it's true that relatively
non-toxic ammonium predominates such pHs, nitrite toxicity _increases_ at
lower pHs. Why? Because nitrite and nitrous acid exist in an equilibrum
similar to ammonia/ammonium. Nitrous acid predominates at pH below 7 and
is much more toxic than nitrite, because it is non-charged and therefore
does not require ion-transport channels, its passage through the gill
epithelium is not inhibited by competing ions (such as Cl-) etc.
pH below 7 is not necessarily safer: while it's true that relatively
non-toxic ammonium predominates such pHs, nitrite toxicity _increases_ at
lower pHs. Why? Because nitrite and nitrous acid exist in an equilibrum
similar to ammonia/ammonium. Nitrous acid predominates at pH below 7 and
is much more toxic than nitrite, because it is non-charged and therefore
does not require ion-transport channels, its passage through the gill
epithelium is not inhibited by competing ions (such as Cl-) etc.
I know I've come across that before.
Yep... from Dr Tim at Marineland:
http://dataguru.org/misc/aquarium/waterinfo.html#Nitrite
QUOTE
NitrIte exists in two forms, nitrite and nitrous acid. At lower pH values (below 8), a greater percentage of the total nitrite exists as nitrous acid (which is the toxic fraction). at the pH values commonly found in tropical fish aquaria (6.5 to 8.0), the majority of the nitrite is in the toxic fraction.
So lower pH helps during the ammonia part of the cycle, but hurts for the nitrIte part of the cycle. need to verify the part about nitric acid uptake thru the gills and salt.