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Re: generating HNO3

Jonathan K wrote:

> Paul Sears wrote:
> >As Steven says, if the KH is 2, you have no problem, and you
> >can probably use less.  What a large KH _does_ protect against is the
> >effect of adding strong acids.  If HNO3 is generated in your tank,
> >then it will use up the KH, and when it runs out......
> Under what conditions would nitric acid be generated in a tank?  Is this due
> to the natural hydration of our ever-present nitrate ions?

Nitrate doesn't hydrate at normal pH.  Normally HNO3 gets added through
bacterial nitrification.  The overall reaction in nitrification could be
expressed as:

	NH3 + 2(O2) -> HNO3 + H2O  (stoichiometry.  I should be ashamed!)

For every nitrified molecule of ammonia you get a molecule of nitric acid.
17 milligrams of NH3 becomes 63 milligrams of nitric acid.

If you have a pretty standard aquarium setup with a nitrifying filter and
you don't change the water for a long time, this constant addition of
nitric acid will consume your buffer; pH will eventually crash.  If your
buffer capacity is low, it can happen fairly quickly.  If your buffer
capacity is high it takes longer.  If you feed heavily or fertilize with
ammonia the problem is worse.

Adding KNO3 doesn't affect the buffer; it's the H part of HNO3 that does
the dirty work.


> I did notice, however, that the KH in that aquarium
> falls noticeably after about a week (I do weekly water changes).  I assumed
> that this was due to the snails removing calcium carbonate from the water to
> make their shells.  Is it more likely that the buffer is being used up by
> HNO3?

If the snail population is growing, then that might contribute to the
drop in alkalinity.  But nitrification is probably the common cause of
KH drops and pH crashes in aquariums.

I doubt that adding KNO3 contributes to stress on your fish.  Could the
temperature be too low?

Roger Miller