fffH? Ffhah? What's that? Oh, like shampoo? Does it have pH in it? What is pH? Who the dickens cares? Why is it that this "window" into water quality (chemistry/physics) is such a powerful tool/consideration? Here's my version of what everyone should basically (pun intended) know about what pH is, it's importance & what you can & should do about it.
pH is a Measure. The symbol pH is a contraction standing for pondus Hydrogenii (=the "weight of hydrogen", the simplest element); pH is the relative measure of hydrogen ion concentration. Put another way, pH is the relative presence of H+ (Hydrogen ions) versus OH- (= Hydroxyl ions). Note when these two types of charged species get together they form an uncharged water (H2O) molecule.
You finally will understand this. In a "pure" solution of nothing but water there is a concentration of @ 10-7, that's ten to the minus seventh (0.0000001), or one in ten million molecules of water that on average have fallen apart/& are getting back together. The fancy mathematical expression for pH is that it's the negative (or one over, the reciprocal) of the logarithm (base 10) of hydrogen ion concentration, or
-log10 [H+] or 1/log10 [H+]
In our example of "pure" water this is log10 1/[H+]*. -log10 [10-7] = log10 [1/10-7] = log 10  = 7
or the log of one over 10 to the minus seven which is a pH of 7.000. Ahhhh. Yay!
Allthat pH is is a measure of other "stuff" dissolved in the solution that affects (net increases/decreases) the concentration of Hydrogen (&/or Hydroxyl) ions. Making the average number of hydrogen ions greater (e.g. 10-6, 10-5, less 10 to the minus negative) means the pH is lower (!) See? Or more acidic (by definition). Going the other direction, having less concentration of H+, meaning the 10 to the minus power is more negative, denotes an alkaline, or basic condition. As an example, consider a hydrogen ion concentration of 10-8 = a concentration of 0.00000001 H+ ions, denotes a pH of 8.000. A 7.000 situation is termed a neutral pH. Makes sense to me.
Maybe one last example (or two). Let's say the hydrogen ion concentration is 10-7.8 or 0.00000078. What is this solution's pH. That's right, 7.8, like much of our beloved southwestern "liquid-rock" tap water. Note that in the above expressions the change between a pH point, let's say seven and eight represents a
change of an order of magnitude, or ten times. Much like the logarithmic scale (Richter) used to describe earthquakes, a small difference in number represents a large change in hydrogen ion concentration. Going from a pH of 6.5 to 4.5 is a difference of 100 times less concentration! This is a big difference, and you should be aware and wary of the logarithmic nature of the pH scale.
I'll leave it up to the Big Editor, Don Dewey, to leave in/out mention of the upper/lower limits of dissociation of acids and bases, pH's of 0 to 14 under conditions of complete/full dissociation.
Reserve/Resistance to pH Change: Alkalinity and Acidity
The capacity of a system to resist an upward (example 7.0 +) or downward (less than 7.0) pH is termed Alkalinity and Acidity respectively. The same quantity is in turn referred to as alkaline and acidic reserve. Read those last two sentences again, carefully. This understanding seems easily lost amongst aquarists.
In actual practice what happens is there are materials, chemicals suspended and dissolved and capable (like gravel, coral...) of dissolving (or going into solution), one's added intentionally as foods, treatments, source/tap water constituents that "fight" or resist change in pH. This is what we mean by buffering. Buffers aid/thwart efforts in changing chemistry/physics by resisting shifts (in pH in this case). This is the reason Alkalinity/Acidity tests must be applied in conjunction with simple pH, if much adjustment is needed/desired and/or you're dealing with expensive or sensitive livestock. There may be so much buffering capacity at different levels or points in the system's pH make-up that you may accidentally overshoot and drastically/tragically raise or lower the pH too much too soon. I cannot but begin to tell you how many times I've seen this happen. READ ON!
So all this being said and done, or at least written... so what? Allow me to present this pictograph:
Graph showing the relationship between pH and optimum efficiency:
7.0 7.2 7.4 7.6 7.8. 8.0
In this graph I am trying to describe a hypothetical (but real and practical) relationship between a physiological function (like an enzyme system) efficiency and a range of pH. In the example you can see that peak efficiency is achieved only within a narrow limit, with function dropping off quickly at a slightly higher and lower pH.
This is the Real World! Fishes (& us!) are made up of these transient collections of enzyme systems; fundamentally that's what life is (at least one definition). These systems are affected (pro & con) by suitable 1) points, 2) limits & 3) slow changes in pH.
Maybe illustrating human blood pH phenomena is a good idea. Normal, acceptable range is something like 7.35 to 7.50 (pretty narrow, eh?); slightly basic. When/if you slow/stop ventilating your lungs, blood pH dips in sync. with build-up of carbon dioxide (CO2) in solution as carbonic acid. Alternatively, hyperventilating will blow off CO2, driving-up pH. Both conditions have their practical limits. Your blood has only so much buffering capacity and moderating mechanisms. At some juncture in moving your blood pH too much too fast you will stop, hopefully by ultimately only stopping doing whatever is causing the emergency shift or passing-out at which point your body will unconsciously shunt blood only to vital functions and allow restabilization.
Fish and other aquatic life are not as fortunate sometimes. Their metabolisms are often closely tied with the chemistry and physics of their surrounding medium (water). Too much, too soon shift or deviation in pH is dangerous! Fishes do have buffering mechanisms to resist these changes, especially sudden shifts in pH, but be warned! Healthy, conditioned livestock can weather such changes, but it will/does weaken them; & less-than-prepared organisms may be pushed over the edge. The challenge of ph shock may not/usually is not obvious at the time if the individuals survive, but other negative influences can/may cause their demise "mysteriously" within hours to days to weeks.
Certain events are particularly important to be on guard for pH shock; from system differences, pH adjustments, and "drift". Permit me to elaborate; I'm going to anyway.
In small volumes of water with lots of metabolic activity (lots o' critters respiring, excreting, secreting) pH buffering capacity is successively eaten away. Ensuingly pH itself is lowered and possibly a critical error will be made in immediately transferring livestock from a CO2 induced lower pH/high ammonia laden shipping condition to a "normal" or at least different, higher pH system. What can/should you do? #1, check pH of the system you're moving from and to and ameliorate the difference. If there is any measurable ammonia with a large pH differential, DO NOT DARE MIX THE SHIPPING WATER WITH THE SYSTEM WATER!!! The combination of dramatic, sudden, higher pH shift is deadly.
When presented with circumstances/conditions when pH shock is unavoidable, shock in the positive direction, where the stock will and should be.
What's a "Good" pH, & Should You Shift It?
Within a range of optimization and maximized point and range of pH it is important to stress that a stable point and narrow range are just as important as any given pH. Therefore the statement: adjusting the pH purposely, outside of just making frequent partial water changes is not suggested.
Over time with the addition of foods, operation of filters, alkaline reserve is nicked away at levels and pH subsequently falls.
Your task is to guard against drastic, fatal drops through monitoring, mixing in of stabilizing chemicals, either through outright addition, or better still via dilution/supplementation through the aforementioned water changes.
Maybe one mention of conditions/circumstances that call for intentionally modifying pH (among other parameters): breeding & treatment for disease, especially environmental. That's it.
Should You Manipulate pH?:
Again, generally no. Many more dollars in wet-stock are lost (bumped-off) through mistakes in attempted adjustment then from given conditions. Beware of fast and overt changes in water chemistry and physics. Shades of Lincoln's Farewell Address!
Measuring pH: Means and Efficacy
The two most popular/common ways of determining pH point and alkalinity/acidity involve colorimetric and electrometric assays.
The latter are the electronic pH pens, meters et al. that are mainly the realm of the chemist and advanced aquarist/tinkerer. Colorimetric tests are simpler, cheaper and accurate enough for most all of us mere mortals.
Liquid, "dry-tablet", and "pillow-reagent" reagent varieties abound. All will do. I suggest assessing your long-term needs and getting an inexpensive and sophisticated model. Having standards set in permanent plastic is a bonus versus being printed on paper. The cheapo kit can be utilized for inexpensive "any day" testing and the better alternate you can use for checking on the former's precision and for back-up. Okay? Ho-tay!
Oh and yes, unless you possess infinite patience, a talent with crystal balls, lots of cash, an S and M involvement with your hobby, or a combination of the above, buy and use an alkalinity/acidity test kit. You will not, cannot alter your system effectively without the information gained from these tests. For those who have kept or been kept by pools/spas, there really are such things as assays (tests) for alkaline/acid reserves rather than just pH point. If you have poorly buffered water (which you will find out through these tests), or over-crowded, over-fed conditions or where you're keen or need to be for commercial breeding, such tests are a very good idea. Enuff said/written.
How to Shift pH: If You Must!
Let's divide into two basic ways: Organic and In (or non-) organic.
Organic pH downers we've referred to in terms of just letting time go by with foods/fish "wastes" accumulating. There are organic acids (e.g. tannins, humic acids) that are not "strong" or concentrated acids depending on the source. Considering the make-up of the alkaline reserve and your specific needs/desires addition of these extracts is the next-most appropriate method to rapidly or permanently reset the pH lower.
Inorganic methods include dilute phosphoric and other acid solutions, carbonate and bicarbonate solutions and solids among others. Some are mixed together to render a liquid or solid "cocktail" that will keep the system's pH among other criteria, within a certain range. Yes, I'm making reference to the various homemade and commercial "stabilizing blocks, pills and tablets". These are unnecessary unless you have source water with no (desired) buffering capacity and/or poor maintenance program.
Where/when in doubt, add some check pH, add a little more....
Some Mention of Factors Affecting pH:
1) The on-going reductive nature of captive systems has been mentioned.
2) Aeration can significantly, temporarily raise pH.
3) Photosynthesis from plants and algae may erratically drive pH and be dangerous in the absence of stabilizing buffering mechanisms.
4) Ozone (if you use it) can drive pH up quickly.
5) Carbons and other chemical filtrants can shift pH if used too much, too soon, too late.
6) Temperature: Hydrogen ions dissociate more readily at higher temperatures. pH does drop some as temperature increases (in the absence of buffering). Not a worry/consideration if your temp.s are relatively/practically stable, blah, blah, blah.
Ever open your eyes while flush your face with water of a wide-differing pH? Not a good feeling in many municipalities. Imagine being immersed twenty four hours a day! No thanks.
Remember; this is only an elementary review. pH affects and is affected by many other factors important to aquatic life keeping. Be aware of your water quality, if nothing else, at least pH. Keep it more or less stable and optimized through modifiers and regular maintenance.
Cuny, Joe F., 1986. Hydrogen Positive. Koi USA, AKCA.
Dewey, Don, 1978. Aquarium Test Equipment and How To Use It, Part I: pH. FAMA 1/78. Part VII: Alkali Reserve. FAMA 8/78.
Dow, Steve. 1983. Le Chatelier's Principle and pH. TFH 8/83.
Halliday, Robert W. 1980. pH. Parts 1, 2, 3. FAMA 7, 8, 9/80.
Kehoe, T.J.: "pH and pH-Measuring Systems," in Process Instruments and Controls Handbook, 2nd edition (D.M. Considine, editor), McGraw-Hill, New York, 1974.
Mowka, Edmund. 1988. Understanding Factors that Affect pH, & Guide to Alkalinity and pH Control. Seascope. Aquarium Systems, Volume 5, Fall 1988.
Thomas, Andrew and Jim Purcell. 1989. Electronic pH Measurement and Control. FAMA 7/89.