The pH scale, which describes the acidity or alkalinity of a liquid can be confusing! The units on the scale run from zero which is extremely acidic, through fourteen, which is very alkaline. In the middle, around 7.0, is the neutral, neither acidic nor alkaline. The numbers on the pH scale measure the concentrations of Hydrogen ions [H+] in the water. More precisely, pH represents the negative logarithm of the concentration of hydrogen ions. Thus, the higher the pH (the more alkaline the solution), the lower the concentration of hydrogen ions. For example, a solution of pH 8.0 has a H+ concentration of 10 -8 (1 X 10 -8 = .00000001). Thus a pH of 6.0 has a H+ concentration of .000001. OK enough about chemistry class. pH preferences of Live-bearers (guppies, swords, etc.) is 7.0 - 7.4. For typical egg-layers (tetras, catfish) its 6.5 - 7.2. For African Cichlids its 7.6 - 8.4. For Salywater fishes its 8.0 - 8.4. And for most marine inve3rtebrates its 8.2 - 8.4.
Filling your tank with water of right pH is only the first step, because the presence of living organisms in your tank causes its pH to drop steadily. Also, the waste in your tank (ammonia) as beneficial bacteria changes ammonia into less toxic material, they release hydrogen ions. Also, if the tank is overcrowded, carbon dioxide given off by fishes and bacteria will build up in solution and acidify the water.
Finally, what does all this mean? Chage your water with new tap water knowing/testing it first. If the pH is in your required range, do nothing but moniter it weekly. API makes solutions to rise or lower the pH. I do not really recomend this, just keep the pH as close to the range in your tap water. Hope this all makes sense.
