PH problem

[FC]

You should check the tap water, not the tank's water. Your tank's water require "reset" or large water change.

[fabry]

Hi Fabrizio, could you elaborate further (in layman's term, please) why this is so. Is there no natural occurence where water is both acidic and hard?

My PH level was 6-6.5 now but the KH now drop to 0,

Hi Ronnie and Ming Han.
Just a little preface that could help someone before answering your question.

Carbonate hardness (would be better to call it Alkalinity or water buffer capacity) is a part of a buffer system including Carbonic Acid (from CO2) and some of its salts (like Calcium/Potassium/Sodium/Magnesium Bicarbonate).

This buffer has the capability to contrast ph fluctuations.

To explain how this happens, let us consider the following example in which we add some Hydrochloric acid to a solution of Sodium Bicarbonate.
Putting it in a simplistic way we could say that:

NaHCO3 + HCl = > NaCl + H2CO3 => NaCl + H2O + CO2.

Which means that the Hydrochloric acid, stronger than the Carbonic one, displaces the latter from its salt.
Carbonic acid has then the tendency to break up into water and CO2.
CO2 is taken up by plants thus remaining NaCl + H2O.

As you can see no trace has remained of the H+ ions we added (the acid) AND of the bicarbonates originally present into the water.

Therefore we can add acids (ions H+) until we have bicarbonates (NaHCO3 in this example) without affecting the pH and producing the only effect to decrease alkalinity (KH).

If we continue to add acids sooner or later the bicarbonates/carbonates will disappear and the H+ ions of the acid will remain ‘free’ into the water thus causing the pH to collapse (possibly killing plants an fishes).

Thus soft water (low Alkalinity/buffer capacity) is easier than hard water to have fluctuations of pH if some sources of H+ (or OH+) are available.
Generally in nature we have sources of H+ ions (acids coming from decomposition of organic matter – carbonic or fulvic for example).
The most present in the waters we need to consider is by far Carbonic Acid (CO2).

CO2 content in a stream/lake is mainly dependent on the amount of organic matter that decomposes in it, so we can consider first that it is a given value (pretty stable and dependent on exogenous variables) and take a look at the table in the link below (sorry for the poor quality):



As you can see, given the concentration of CO2, the pH depends on water alkalinity.
For example if we have 30 mg/l of CO2 the pH can be 6.6 at a KH of 4, 6.4 at a KH of 2.5 or even 6.0 at a KH of 1.

Hence softer waters have also a lower pH in comparison to harder waters that have also a higher pH.

Of course if someone dumps in the water some amounts of organic matter this will be decomposed by bacteria producing CO2 and lowering the pH (again considering the same KH) even further.
So, one of the very few possibility to see a water with high Alkalinity (KH) and low pH is to have a source of CO2-Carbonic acid into the water (thermal, decomposition or whatever).
Acids other then carbonic, like the humic-fulvic acids coming from peat and humus will lower the pH too, but will also lower KH accordingly.

All this means two things for aquarium-keepers:

1) There is no need nor utility to go against nature trying to have a low
pH with a high KH in our tanks.
2) Trying to decrease pH with acids different from Carbonic acid (CO2)
without affecting (decreasing) consequently also KH is totally hopeless.

Hope this helped Ming Han and someone else.

Regards