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Re: pH changes with 'Nitragon' filter

On Thu, 13 Jan 2000, Kevin Buckley wrote:

> The interesting thing is that it also reduces the pH of the tap water to <7.
> 3 (the low-end of my high-end test kit) which resulted in a tank pH, after a
> 50% water change, of 7.6 (the tank Nitrates also dropped to around 40 -
> 60ppm).  This is not an advertised feature!

I'm not familiar with the 'Nitragon' filter, but it sounds like an anion
exchange resin.  Unless the resin is highly selective for nitrate then
it will also make some other changes in your water chemistry.  It sounds
like in addition to removing nitrate, it may also remove bicarbonate -
either directly or indirectly by releasing hydrogen ions that attack the

If so, then you can compare the alkalinity (KH) of the mains water to the
alkalinity of the water coming out of the filter.  There should be a

> The thing I don't understand is that when I re-tested the tank water, prior
> to doing another 50% water change about a week later (I'm trying to quickly
> get the Nitrates down to around 20ppm initially & then I plan to try a
> Nitrate removing resin in one of the filter compartments), the tank pH had
> risen back to 8.2.
> How could this happen?

It's hard to know what is going on in the dynamics of your tank.  There
are probably several possibilities.  The simplest possibility I can think
of is that there are shells (maybe snail shells) or other sources of
carbonate in your tank.  The carbonates dissolve into the water, raising
the KH, and the pH increases because the KH is higher.  The fact that it
returns to the same pH value that it started with may reflect the balance
between the carbonate source, the alkalinity and hardness of your water
and the CO2 generated by respiration in your tank.

Roger Miller