There are two main factors that can reduce the alkalinity ( or reserve buffer substances):
1) The nitrification of food and organic waste.
2) Precipitation of carbonates by filtration materials of limestone origin (Cal).
The first process, the nitrification of food and organic waste is the main work of the biological filter. Food is processed through the fish in the aquarium. producing toxic ammonia, which is optionally reduced by the filter in relatively non-toxic nitrate. The side effect of this necessary process results in the production of acid. This acid reacts with the bicarbonate ions to eliminating them progressively. Thus the rate of alkalinity (or reserve buffer substances, commonly known as the buffering capacity) is slowly degraded and the pH drops.
The second process is related to the type of substrate used for filtration. Materials like crushed coral rocks, the dolomitic limestone (dolomite) and crushed oyster shells, especially when new, tend to attract calcium carbonate (calcite) to their surfaces. As a result they are able to remove it from the water by precipitation. The speed of this process depends on the type of material, age, and certainly of their origin.
Because of highly active materials, such as some coral ground, the alkalinity can drop by 50% in four days or less, with the resulting drop in pH. The effect diminishes over time as the contact surfaces of the substrate are covered with crystals and organic sludge. If thereafter the buffer substances are replaced, or by a change of water, or by addition of buffer substances, the alkalinity will return more stable. In the meantime there will be a significant drop in pH, even with little or no fish present.
Hope this is clear and sorry for my english, I'm French
