Alkalinity and pH are distinctly different from each other, although their definitions and functions can be easily confused. Alkalinity is essentially a measurement of water’s ability to neutralize acids. It’s a measure of the buffering capacity of a system while pH is basically the measurement of the concentration of hydrogen ions in water, in terms of acidity or alkalinity. The alkalinity of water regarding pH issues merely refers to the basic end of a pH scale (alkaline) in contrast to the acidic end of the scale and does not reflect the buffering capacity of a system. It’s easy to believe that water with alkaline pH is likely to be high in alkalinity (buffering capacity). However, this is not necessarily true. Water with a high pH, but a low alkalinity is regarded as unstable. Such water will quickly decline in pH with the natural accumulation of organic acids in aquarium systems.