pH and Buffering

Unfortunately the Ph of water is not constant and is always changing.
The equilibrium of ions in water is very delicate and it is observed that the Ph of water moves up or down on its own.

RO or distilled water in the absence of air reacts neutrally with a Ph of 7.
Yet when water is permitted to stand you will observed that a Ph value sides down to about 5.3

This is caused by the carbon dioxide in the air, part of this combines with the water to form carbonic acid.
The salts of carbonic acid the carbonates and bi carbonates are important for the stabilisation of the Ph; they are Ph buffers which mean they prevent the Ph dropping.

Electronic pH meter

Discus fish keepers setting up tanks with very soft water < 5 Gh need to watch the Ph very closely. Addition of marble or limestone can be useful (small amounts) as this can release carbonate hardness over time.

The calcium carbonate (limestone or marble) leads to the formulation of calcium ions which may also be picked up by the fish for building bones.

Nitric acid is formed during the nitrogen cycle (nitrification) in discus fish water which is the reason our Ph drops off over time. This acid can be neutralised by the carbonates in the water. The end product of which is Nitrates.

So as can be seen in a tank the Ph may drop off due to the formulation of nitric acid in the nitrogen cycle which in turn will make the Kh drop as this is neutralised to form nitrates, thus  leading to this fall in Kh  and hence the buffering capacity of the water.
If left unchecked all the carbonates are used up and the Ph may drop further and rapidly due to the formulation of nitric acid and no buffer available to turn it into nitrates.

Quick buffering of a rapidly falling Ph water can be achieved by adding sodium hydrogen carbonate (sodium bi carbonate or baking soda to those who cook) to the water.

This must be dissolved in water first an then added SLOWLY  in SMALL qualities over a period of time testing the Ph regularly as you go. This process goes on till the Ph is stabilised and returned to the desirable level (6.5 to 7).

If the Kh (carbonate hardness) or Gh (total hardness) of discus fish water is to be lowered this can be achieved by the addition of RO water over a period of time. The water changes should be moderate & must be done slowly at regular intervals.
If too much water is changed the water parameters can change dramatically and become unstable.
If not enough water is changed the waste from the discus fish may not be removed.