Ph is an ongoing concern for reef keepers and aquarists. Typically it is a concern more focused towards reef tanks because fish tanks can tolerate a larger flux in Ph levels before causing irreversible damage. In contrast a reef environment must maintain a constant Ph level in order to sustain life. The best Ph level in a reef tank ranges between 7.6 and 8.4 and best suited to the higher side of that range. The normal behavior of Ph in a saltwater tank tends to be on the acidic side (low) Here are some reasons for this.

1)Nitrification
2)Too much carbon dioxide
3)Natural acids created by metabolic waste

Sea water contains natural buffers like bicarbonate, borate, hydroxide and others that restrict any drop in Ph levels. One easy fix for a drop in Ph that is often used is baking soda which contains bicarbonates. Another dreaded method is to do a water change, this should replenish the natural buffers and minerals in your tank. One big reason for a drop in Ph is left over food and fish excrement so removing these things from time to time could be of some benefit.

From WikiPedia:

The pH of seawater plays an important role in the ocean's carbon cycle, and there is evidence of ongoing ocean acidification caused by carbon dioxide emissions.^[27] However, pH measurement is complicated by the chemical properties of seawater, and several distinct pH scales exist in chemical oceanography.^[28]

As part of its operational definition of the pH scale, the IUPAC defines a series of buffer solutions across a range of pH values (often denoted with NBS or NIST designation). These solutions have a relatively low ionic strength (~0.1) compared to that of seawater (~0.7), and, as a consequence, are not recommended for use in characterising the pH of seawater, since the ionic strength differences cause changes in electrode potential. To resolve this problem, an alternative series of buffers based on artificial seawater was developed.^[29] This new series resolves the problem of ionic strength differences between samples and the buffers, and the new pH scale is referred to as the total scale, often denoted as pH_T.

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