. pH comes from the French word "hydrogene" which means hydrogen power. It is basically a scale from 1-14 that equates to the ratio of hydrogen ions to hydroxide ions in water. 7 is neutral so they are present in equal amounts. Below 7 is acidic and there are more hydrogen ions. Above 7 is alkaline indicating a higher concentration of hydroxide. The pH scale is also logarithmic. By this I mean that pH changes by a factor of 10 with each step ie: a pH of 6 is 10 times more acidic than 7; a pH of 5 is 100 times more acidic than 7; pH of 4...1000 times more acidic....forget 3, I can't count that high. pH also influences how other substances react. Ammonia, for instance, is much more toxic at higher pH's while chloramine (often found in city water) is more toxic at a low or acidic pH. In tanks with excess waste products, the production of carbonic acid will often lower pH. Likewise, if your tank has a very high concentration of nitrates, the nitric acid that is present will lower pH. Now, it's hard to talk about pH without briefly discussing hardness or gH. This is the concentration of calcium and magnesium ions that are present in your water. If these are present at a high level, you have "hard" water. Hard water is typically above 7 or alkaline. But...not always because we also have kH or buffering capacity. This is known to us fish nuts as carbonate hardness. Basically, this is the concentration of carbonate and bicarbonate ions. These will act upon the pH by neutralizing or buffering any added acids. kH will not normally change pH. Be careful about using distilled or pure R/O water in a tank. This water has very little or no buffering capacity and will drop pH very quickly with the addition of very little acid. There are many commercial preparations that will set the proper hardness and buffering capacity in this water. I guess the basic point I'm making here, in addition to my feeble attempt to explain pH, is that darn near everything in the aquarium interacts in some way with everything else. You may set your pH just to where you want it and come back 2 days later to find a very different value due to other substances in the aquarium that have acted upon it.
