pH

This is a measure of the acidity or alkalinity of water. Perfectly neutral water has a pH of 7.0. At pH 7.0 water has an equal and tiny amount of both acidity and alkalinity. If an acidic substance is added to neutral water (such as pH down) it will result in a lower pH. At pH 6.0 the concentration of acidity is ten times as high as at pH 7.0. At pH 5.0 the concentration of acidity is ten times as high as at pH 6.0.

Thus, in pure water it takes more acid to lower the pH from 6.0 to 5.0 then it took to lower the same water from 7.0 to 6.0. Our aquarium water is not pure water though. It always has some concentration of dissolved materials in it. Some of these dissolved materials can act as so called buffers. A buffer simply causes the system to resist changing pH when an acid is added. Common buffers found in aquariums are calcium and magnesium carbonates and bicarbonates as well as sodium phosphates.

So, often when you add pH down to make a tank more acidic it may take a lot to get a small initial change then suddenly a small amount can have a larger effect. The initial high demand is simply the amount needed to overcome the natural buffers present. About the only time tanks have little natural buffer present is when they are filled with either very soft water or reverse osmosis water.

It is actually easier to maintain pH on the slight alkaline side then on the acidic side as there are minerals that you can add that automatically consume any acidity that develops in the tank. These materials are all forms of calcium and magnesium carbonate. Examples are limestone, dolomite, tufa rock and shells. In a typical rift lake tank or salt water tank you want a pH of 8.0 to 8.5. A generous layer of such material on the tank bottom will hold the pH in the desired region fairly automatically.

Hardness

Hardness is simply a measure of the dissolved salts present in water. Hard water has larger amounts of dissolved salts then soft water. Most well waters in this area as well as Lake Erie water are medium hard. Rain water, RO water and to some extent peat treated waters are soft. The majority of the hardness chemicals present in local water are calcium and magnesium carbonates and sulfates.

Most fish evolved in an environment where both pH and hardness were fairly constant. This is particularly true of salt water fish. Our oceans are so hugh that they have very constant hardness and pH. Other then fish that live right in the effluent of rivers and streams that discharge into the ocean most salt water fish have very low tolerance for changes in either hardness or pH.

This tends to be less true of freshwater fish as they live in less stable environments for the most part. Still, each fish has its own preferences that typically reflect the conditions where it is native. Thus, for many of the fish we keep some adjustment of either pH or hardness or both is needed for optimal success.