Acidity/alkalinity: what is the pH of your water?
What is pH? Most briefly, the pH (exponens Hydrogeni, though you'll read other interpretations) measures the "negative exponent of hydrogen" ions. The pH scale runs from 0 (acid) to 14 (base or alkaline), as you know from doing pH tests. The scale is set up by figuring the weight of the hydrogen ions in a liter of water (at 20°C). The weight of the hydrogen in a liter of absolutely pure distilled water is 1/10,000,000 gram. Instead of all the zeros you can just say "ten to the minus seventh power" or use the inverse logarithm 7 (i.e. 7 zeros) for the exponens Hydrogeni, that is for the "p" of the "H". Hence a pH value of 7.
By comparison, water at pH 6 has ten times as much hydrogen, that is 1/1,000,000 gram ( i.e. one over only six zeros). Because the pH scale is logarithmic, a drop from pH 6.0 to pH 5.8 is ten times greater than a drop from 7.0 to 6.8. Ten times more stressful for your fish.
As you get to an acidity where the weight of hydrogen ions approaches one gram (a pH of 0.0) the scale becomes increasingly theoretical. In the real world, Pepsi-Cola is about pH 3, what with phosphoric and citric acids plus all that CO2 diffused into it under pressure; but next morning the "flat" Pepsi is much less acidic— and much less refreshing!
Fish maintain a blood pH that is just on the alkaline side of neutral, about pH 7.7. They are sensitive to wide, abrupt changes in the pH of the surrounding water. So, don't shift the pH more than two points, that is, 0.2 degrees, at a time. Now, everyone has a favorite number attached to this warning! but the core message is that— within reasonable limits— pH stability is what counts for fishes, not an "ideal" pH value.
Alkalinity, the "buffering capacity", measures any of the substances dissolved in water that can take up or release the H+ ions that affect pH. A well-buffered water exhibits a stable pH; a too-lightly buffered water may experience a drop in pH, which might be what you want, say for "blackwater" fishes. Alkalinity rates its own discussion.
Adjusting pH values can be tricky and rarely leads to long-term success and stability. Sometimes the motivation for lowering the pH is to breed fish that are said to require a pH below 7.0. Attempting to adjust the pH in water that has a strong carbonate buffer can feel like trying to submerge a beach ball. Phosphate pH buffers are dis-commended in planted aquaria, even by the manufacturers. See the results of a search for the ingredients of Aquarium Pharmaceuticals' "Proper pH7.0" in the Skeptical Aquarist's material onwater conditioners.
Soft tapwater combined with high pH can be an obscure, counter-intuitive headache for some fishkeepers living in areas where municipal water originates as groundwater drawn from limestone aquifers. In such waters, when the sum of dissolved calcium and magnesium exceeds about 150 ppm, large municipal water boards may resort to a water-softening technique called the "lime/soda" method in order to precipitate some of the minerals, thus softening tapwater right at the plant. The technology of lime/soda water softening is part of the water softening discussion.
Recent experiments are showing that environmental cues for fishes' gonad maturation are connected to decreases in water conductivity rather than to low pH values themselves. Reducing the total dissolved solids (TDS), as a spate of rainwater would do, is a better way to go; some methods for water softening have their own page in the "Filtration" folder here.
The balance between KH and CO2 results in pH. The pH can also be considered as the result of the balance between the CO2 level and the alkalinity or "KH" ("carbonate hardness"). Dissolved CO2's effect on the pH is a subject that needs its own discussion.
"pH shock." What used to be called "pH shock", actually "ammonia shock", as non-toxic ionized ammonium (NH4) is changed to toxic ammonia (NH3), is a result of suddenly restoring the buffering with a large water change, after pH has been slowly depressed by bio-acidification; it's discussed at that page.
Some pH links. If you're willing to work through it, the best basic chemist's answer to the question "What is pH?" is found at "General Chemistry on-Line." By the way, this site is good for definitions of any basic chemistry concept. Sometimes it can clarify even my murky brain.
The best fishkeeper's introduction to pH that can be found on the web is at The Krib once again. The best explanation in an aquarium-related book is Diana Walstad's, in The Ecology of the Planted Aquarium, 1999, but behind her book and informing her understanding is the great standard college text, Dr. Robert G. Wetzel's Limnology: lake and river ecosystems, which really does explain the pH of fresh waters.