Water is crucial for all life--most of us know that and don't take it for granted. But just imagine if you lived in water your entire life as a fish does, water would be even more important. Beyond just swimming in it, you would depend on it to extract all your oxygen. It would move in and out of your body constantly and you'd have to work to keep your fluid and salt balance in sync with it. You would excrete all your wastes directly into the same fluid you are taking in. The unique watery environment fish live in and the balance they must maintain with it make fish unique as pets, and in some ways, more difficult to keep than some other pets.
Most people get a little hazy-eyed when they hear the words 'water chemistry'. "Oh God", they say, "I just want a few little fish in a simple tank, why do I have to go back to all this confusing stuff I never got the first time around in high school". But the truth is, just understanding a few very basic things from the beginning can save you tons of trouble and heartache later, since these are all factors your fish needs to deal with every second of their lives. And really, it doesn't have to be all that complicated, if we can break it down into simple terms.
Let's start with tap water, which is what most people use. This is usually more than just H2O (water molecules) alone. Dissolved in this water, at low concentration, are other ions (charged molecules), gases, and other compounds dissolved in it at low concentration (specific concentrations depend on your area, every place is different, though the EPA keeps a watch over maximal levels of everything). In tap water, some of those include chlorines and chloramines, compounds added in that discourage bacterial growth. However, they are also potentially irritating or toxic to fish membranes, so we use dechlorinator to inactivate them and make tap water safer. Chlorine also evaporates slowly over time as chlorine gas, so in the "old days", people that kept fish would just get their water out of the tap and let it sit for 24 hours.
There are also other potentially dangerous impurities that are found in tap water at very very low concentration (many of them are positively charged, and so can actually raise pH, a subject discussed below). With most FW fish, this is not usually a real issue at these low levels; they aren't that sensitive to the parts per million of trace impurities (such as copper, zinc, phosphates, etc.). However, for some very sensitive FW fish (such as discus) and for those who want to have marine tanks with delicate fish, corals and other invertebrates, this can be an issue. These impurities can then be removed by various methods, including reverse osmosis and deionization. Bottled water and "spring" water can use some of the same methods to remove these impurities, but neither RO, DI, distilled nor bottled water should not be used exclusively in the aquarium for the reasons mentioned under "GH".
The exact balance of ions in water is what determines many of the water parameters that we'll now discuss.
pH is an abbreviation for a German term, "potenz [potential of] hydrogen". Basically, it is a negative log scale from 1-14 which measures hydrogen ion concentration (H+ in solution). 7 is considered neutral pH, and less than 7 we consider acidic (higher H+ concentration), more than 7, basic. Every one unit equates to a multiple of 10 in a log scale.
But who cares--what does this mean to you and your aquarium, in layman's terms? Fish are amazing animals in that most can adapt to wide ranges of pH IF GIVEN TIME. The problem many beginners have is that they too hung up on reading about "optimal" pH ranges of different species in profiles, and take it upon themselves to start changing pH with "pH Up/Down" solutions, thinking it will just be a simple matter. DO NOT FALL FOR THIS. These solutions are pure acids and bases that will only cause a temporary but sudden spike or drop in pH.
Eventually, the buffering capacity (discussed below) will push your value right back where it was, but this relatively brief fluctuation will severely stress your fish, in many cases killing it due to pH shock (and in some cases, just stressing it to the point of disease).
Many novice fishkeepers are under the impression that they have to get their pH "perfect" to some ideal value ("I want it at 7.0"), which is not the case at all. Usually, hardy fish can live fine even in between a wide value of 6.0-8.0 if allowed to acclimate very slowly with one water value from the time you first bring them home (floating with small quantities of water exchange and drip acclimation work best). Even fish that supposedly prefer slightly acidic water have been amazingly adaptable to water outside their "textbook" range, (though they have limits)--so long as they are given TIME. You'll just have to trust me on this issue; from working at pet stores in the past, I can say that "pH Up/Down" may be the single-leading product on the market today that leads to fish death through misuse (followed by cases of improper use or overdose of medications).
It is these rapid fluctuations, especially those created by artificial "pH adjusters", that hurt fish far more than being kept at a slightly suboptimal but stable value. This is why in general to beginners, I recommend to MATCH YOUR FISH TO YOUR WATER, NOT YOUR WATER TO YOUR FISH. This is the proper use of pH range information that can be found in species profile care sheets. pH modification should be left only to more advanced fishkeepers who want to keep and breed difficult, picky fish, and are willing to deal with the consequences because they have a thorough understanding of how any one of these water parameters can affect another.
KH stands for carbonate hardness (again, the K is from those wacky German spellings), also known as buffering capacity, already touched on above. This isn't as well known as the term "pH", but is actually the parameter which determines how stable a pH can be. A great analogy to use is to think of KH as a sponge for your hydrogen ions. If a solution is more buffered (higher KH), then this means you have a powerful sponge that can absorb H+ ions and keep the pH from changing much from where it's at. This also explains why simply adding pH adjusters causes pH "bounce".
In the FW aquarium, most of your KH is determined by carbonate and bicarbonate ions dissolved in the water. Most beginners also don't need to worry excessively about this value, but it is just a good concept to get your head around (since it relates to pH). The only time raising KH is really called for with beginners is in the African cichlid tank and beginner's marine tank, both of which require high, steady pH (around 8.2-8.3 for marine, higher for African) and high KH, and a buffer will stabilize these values (unlike a pure acid or base, described above, that does not significantly affect KH). In the marine tank, this should only be done through premixture of commercially available buffers with marine salt (which has many buffering qualities of its own). For the African Rift Lake tank, these cichlids are very hardy, and you can get away with using "poor-man's" DIY recipes of baking soda and epsom salts at the proper concentration. There are key differences (between African and marine tanks), however, as we will soon see below.
This buffering level [in these specialty tanks] can later be maintained through use of different forms of solid calcium carbonate--in the marine tank, live rock, coral skeleton, and aragonite sand; in the African cichlid tank, the counterparts are Texas Holey Rock, other forms of limestone, and crushed coral substrate. Shells may be used in either. You do not want to use any of the above in a FW tank where low buffering, neutral/acidic water is prefered, because those objects will dissolve slowly and raise your KH (not desirable with many ordinary community tank species, which are from softer water, a topic covered below).
GH stands for general hardness (whew, those Germans gave us a break this time with remembering acronyms!). It is a measure of the concentration of dissolved ions of magnesium and calcium. In common terms, it refers to how "hard" or "soft" the water is (though many people mistakingly interchange these terms with the terms "basic" or "acidic--not quite the same, but closely related--those are terms to describe pH with, already discussed above). In fact, the "big 3": ph, KH & GH, are all very intimately and closely related and will change in response to one another, making any kind of water modification, if you were to choose to try it, a very delicate juggling act.
In general, many of the soft water fish we keep come from the Amazon and other rivers around the world that have a relatively high organic content, and so keep KH & GH relatively low (through ion exchange with peat and organic substances like driftwood, but I won't go into that here in detail, I'll just say that those are both natural ways of softening water). These fish do not have a strong tolerance of many dissolved salts (which can be composed of any ions, not just sodium chloride as we know it) that may raise those values.
Hard water fish that prefer water with slightly elevated KH & GH, on the other hand, come from bodies of water which have ions from minerals in the surrounding rock face constantly leaching in to dissolve in the water. Usually these are fish from the plata Yucatan region of Mexico and Central America near the delta watershed regions (most livebearers), or to a much greater degree, hard-water lakes with a lot of rockwork like Lake Malawi and Lake Tanganyika (both in Africa).
NO fish, however, can tolerate 100% PURE water with NO electrolytes/ions in it, which pretty much is what distilled, bottled, or pure, unchanged RO/DI (reverse osmosis deionization) treated water is. Aquariums should never use this water exclusively (without mixing back some needed electrolytes), except for topping off evaporated water, also covered in the next section (Part II).
You can skip this part totally if you only have a FW tank. I include it here only in the interest of completeness and for those with an interest in marine tanks. Salinity includes ALL the dissolved solutes in water, so it encompasses KH, GH and other ions both groups leave out, like sodium. It is measured by a simple mechanical device called a hydrometer, which basically just measures density of the water (which is directly related to dissolved solute concentration at a set temperature). This value is known as "specific gravity" (SG), and is measured in comparison to pure FW (1.000 exactly). Brackish water values usually fall between 1.005 and 1.018, depending on the species. Marine water is usually in the 1.022-1.025 range, though usually more tightly around 1.023-1.024 with no fluctuation.
Please take note here that marine salt has a specific mix of MANY ions, including the big components, calcium, magnesium, and carbonate for buffering, but other trace elements needed by marine animals, such as sodium, chloride, iodide, bromide, strontium, etc. Brackish water should also contain this ratio, just to a lesser concentration (lower salinity).
As such, I wanted to touch on the idea that some people have that adding sodium chloride ("aquarium salt", used for treatment of sick fish, the same exact thing as table salt) is creating true brackishness. This is NOT the same thing. First off, the concentration for treatment would be too low anyway (1 tbsn/5gal) to really make a dent in SG measurements. Secondly, this is not a natural type of "brackishness", because you aren't adding anything except sodium and chloride ions--they don't affect KH or GH. There are no natural bodies of water that are high in only these ions, so there is no reason to add aquarium salt in the healthy FW tank, a real myth that is spread online. It does find its good uses in some sick tanks for a number of reasons (including discouraging bacterial and parasitic growth, reducing osmotic stress to the gills, etc.). However, there are also fish (mostly soft-water, delicate species or species without scales) that do not tolerate it well, and this is also discussed in that article.