When you measure pH you are measuring how many parts of Hydrogen ions (H+) and Hydroxide Ions (OH-) are in the water.
If there are more Hydrogen Ions then Hydroxide ions then the water is said to be acidic and has a pH below 7. If the Hydrogen ions and Hydroxide ions are equal then the pH is considered neutral and has a value of 7. If there are less hydrogen ions then hydroxide ions then the water is considered alkaline and has a pH value greater then 7. The degree of acidity or alkalinity is measured on a logarithmic scale between 0 - 14, 0 being the most acidic and 14 being the most alkaline.
Because the scale is logarithmic, a difference of one pH unit represents a tenfold change.
For example, the acidity of a sample with a pH of 5 is ten times greater than that of a sample with a pH of 6.
A difference of 2 units, from 6 to 4, would mean that the acidity is one hundred times greater, and so on.
In the Aquarium: Fish have evolved in very specific aquatic environments where the pH is generally stable for long periods of time and have very little fluctuation. Most fish have evolved in water with pH generally between 5 to 8, however some fish have evolved in extreme conditions as low as 4.5 and as high as 9.2 and these fish are generally much more sensitive to changes then most tropical fish.
Fish have evolved to match their physiology to their environment in order to make it easier for their systems to function without expending excessive amounts of energy to maintain a balanced internal pH. Fish that evolved in conditions where the pH value is lower then 7 are called acidophiles and those that evolved in environments where the pH is above 7 are called alkalophiles. Most acidophiles and alkalophiles do not live or show their best in water that is on the other side of the pH scale then what they have evolved in. Some species wont be able to reproduce if they are not kept in the proper conditions.
Health Related Issues: A pH change as little as 0.2 major physiological adaptations start to occur. The fish has to use bicarbonate ions or acidic carbon dioxide to order to fight off the effects of the acids or bases that inters it's body. If the change is great enough there can be a deficiency of either one of the compounds in their body and they start to suffer from either acidosis or alkalosis depending on which substance has been depleted.
Although it is possible to keep fish in water with a different pH then they are used to, long term exposure to incorrect pH will cause constant stress on the fish thereby weakening their immune system. Some species wont be able to breed properly, fry can die with any apparent cause, and most fish wont be able to live their natural life span. Most adult fish tend to be able to deal with these physiological changes a lot better then young fish and fry. However there are exceptions to this with fish that live closer to the extreme edges of the normal pH ranges such as wild caught discus and marine fish.
Acidosis and Alkalosis can be brought on in a couple different ways. The first is exposure to pH values drastically outside the fishes natural range for any period of time. A slow rise or fall of pH over the long term has subtle symptoms that can be hard to pinpoint the cause of without a simple pH test to see if they are in their natural range. These symptoms include gasping due to the changes in the oxygen carrying capacity of the blood or from excess mucus production that can cover the gills. Excess mucus production and reddened skin areas. Weakened immunes systems leaving them exposed to other diseases such as bacterial, fungal or protozoan infections.
The second way it can be brought on is through a sudden change in pH even as little as 0.2. Changes in pH should be made slowly to allow the fish time to adapt to the new ranges and should never go outside the fishes natural pH range. Rapid changes in pH can lead to symptoms that include the fish being highly excitable, rapid swimming movements, shimmying, gasping and excessive jumping followed shortly by death.
Acidosis: With most acidophiles that live in pH between 6 to 7, acidosis can become a problem when the pH in the tank drops below 5.5. Most alkalophiles will experience acidosis should the pH drop below 7 and marine fish can show symptoms if the pH drops below 7.5. To treat acidosis it's very important to bring the waters pH up to the fishes natural pH levels and treating the fish for any secondary infections that could have been caused by their weakened immune system.
Alkalosis: Alkalosis generally occurs in pH above 8 for acidophiles or 9 for alkalophiles such as marine fish. To treat alkalosis it's very important to bring the waters pH down to the fishes natural pH levels and treating the fish for any secondary infections that could have been caused by their weakened immune system.
Testing pH: Testing for pH is a very quick simple process and can be done with test kits found in any fish store. It's a good idea to have both a wide range test kit and a narrow range test kit. The first should be used to get a general idea on which narrow range test kit you need for your tank. The narrow range test kit will give you more accurate results that are easier to read then the long range kit will have. There are also digital pH meters that can give very accurate pH readings if your system requires it (such as discus breeding or marine tanks).
Raising the pH: or making the water more alkaline. This will suit Livebearers pH 7.2 to 8, African Cichlids pH 7.5 to 8.4, Marines pH 8.2 to 8.4 and Goldfish pH 7.2 to 7.6. There are a number of ways to adjust the pH up these are :
Lowering the pH or making the water more acidic. Acidic water will suit the majority of commonly kept community fish with the exception of those listed in the previous section.
Make sure there are no Calcareous rocks or gravel in the aquarium.
Fish do not like rapidly changing water conditions. It can bring about their death in some cases so it is vital that any adjustments are made slowly. pH changes should be no more than 0.4 in any 24 hrs.
Most captive bred fish have been in the hobby for so long now that they have evolved to cope with their captive conditions rather than those found in the fishes natural range. This has come about through unplanned selective breeding where the least adaptable specimens died prematurely and the most adaptable went on to breed.
So with captive bred fish it is important to avoid extremes of pH even if those are the conditions found in the wild.
Wild fish however do need to be kept in condition which closely mimic those found where the fish originate from. Most fish are adaptable to some degree but there are limits.