In order to know what a buffer will do to your tank, you have to know what a buffer is. A buffer is something that, when added, will allow the addition of an acid or a base without a change (or a minimal change) in the pH. The more effective the buffer, the more acid or base that can be added and 'absorbed'. In the body, it is the lungs and the kidneys that control the acid/base balance. We use proteins, HCO3, phosphate, and phosphate esters to buffer the addition of acids and bases to our system (H+, HCO3, CO2). A fish tank can't produce buffers, so it must rely on what we add. By adding a buffer to our tanks, we can't expect the pH to change, but to remain constant with the addition of acids and bases. The degree of buffering depends on what we used, how much we used, and to what degree the acid or base is added. In order to change the pH effectively, we would need to add either a basic compound or an acidic compound.