Buffering

In its simplest terms, a buffer is a chemical that stabilises pH. There are lots of chemicals that act as buffers but in fish tanks we need one that won't harm the fish. The most obvious ones are chemicals that occurs naturally in water - in streams and lakes - and that is carbonate and bicarbonate. Fish have evolved to live with those.

In fish tanks we only need buffers that prevent pH drops. Nitrate, the end point of the nitrogen cycle, is acidic. A lot of the waste products produced by the fish are acidic. The natural tendency of fish tanks is to become acidic.

Acids are chemicals that release hydrogen ions; pH is an upside down measure of the amount of hydrogen ions - the more there are, the lower the pH. Carbonate and bicarbonate react with the hydrogen ions forming carbon dioxide and water - I can give you the chemical equation if you want it 

Carbonate and bicarbonate buffer the water against pH drops by reacting with the acids that are continually being made, but as they do this they are used up. In areas with high KH there is so much buffer it would take weeks of no water changes for it to be used up. This is why people with hard water and high KH, who are likely to also have high pH, have great difficulty changing their pH to lower values - all that KH in their water reacts with the 'pH down' type products and stops them lowering pH. But with people who have low KH, there isn't much buffer in the water and it gets used up quickly. I know from experience that if the tank is overstocked and you don't do a water change for 3 weeks (yes, I was guilty of that!) the buffer can get used up and the pH can drop off the bottom of the scale.

Is that any help?