The various parameters and how the affect each other are all detailed below.
The main thing to remember is that the incoming water supply ie: your tap water, varies from region to region and may even vary with the time of year and time of day that the tap is run.
Therefore, the levels indicated below, whilst correct, should be used as general guide lines rather than hard and fast rules.
pH
pH refers to water being either an acid, base, or neither (neutral). A pH of 7 is said to be neutral, a pH below 7 is ``acidic'' and a pH above 7 is ``basic'' or ``alkaline''. A pH of 5.5 is 10 times more acidic than water at a pH of 6.5. Thus, changing the pH by a small amount is more of a chemical change than might first appear.
For aquariums two aspects of pH are important:
1. Rapid changes in pH are stressful to fish and should be avoided. Changing the pH by more than 0.3 pH in a 24 hour period known to stress fish. So you should aim to keep the pH of your aquarium constant and stable.
2. Fish have evolved to thrive in a fairly narrow pH range. You should check with the stockist/supplier to be sure that your aquariums pH matches the specific requirements of the fish you are keeping.
Most fish can adjust to a pH somewhat outside of their optimal range. If your water's pH is naturally within the range of 6.5 to 7.5, you will be able to keep most species of fish without any problems. If your pH lies within this range, there is probably no need to adjust it upward or downward, but keep an eye on it using a test kit of a digital pH meter. Test kits give you less accuracy but cost less, digital meters provide a resolution of as much as 0.01pH and most models compensate the pH reading for the effects of temperature - in the long run they are more economical as, unlike test kits, there are no reagents to buy.
Buffering capacity
Buffering capacity refers to water's ability to keep the pH stable as acids or bases are added. pH and buffering capacity are intertwined with one another. If the water has sufficient buffering capacity, the buffering capacity can absorb and neutralize the added acid without significantly changing the pH. However, once the buffering capacity is used up, the pH changes more rapidly as acids are added.
Buffering has both positive and negative consequences. On the plus side, the nitrogen cycle produces nitric acid (nitrate). Without buffering, your aquarium water pH would drop over time . With sufficient buffering, the pH stays stable. On the negative side, hard tap water often almost always has a large buffering capacity.
If the pH of the water is too high for your fish, the buffering capacity makes it difficult to lower the pH to a more appropriate value. In freshwater aquariums, most of water's buffering capacity is due to carbonates and bicarbonates. Thus, the terms ``carbonate hardness'', ``alkalinity'' and ``buffering capacity'' are used interchangeably. Although technically not the same things, they are equivalent in practice in the context of fishkeeping. Note: the term ``alkalinity'' should not be confused with the term ``alkaline''. Alkalinity refers to buffering, while alkaline refers to a solution that is a base, in other words has a pH greater than 7.00pH.
General Hardness
General hardness refers to the dissolved concentration of calcium and magnesium ions. When fish are said to prefer ``soft'' or ``hard'' water, it is general hardness that is being referred to.
Water hardness follows the following guidelines. ppm means ``parts per million'', which is roughly equivalent to mg/L in water.
General Hardness
0 - 70 ppm very soft water
70 - 140 ppm soft water
140 - 210 ppm medium hard water
210 - 320 ppm fairly hard water
320 - 530 ppm hard water
Salinity
Salinity is usually expressed in terms of its specific gravity, the ratio of a solution's weight to weight of an equal volume of distilled water. Because water expands when heated (changing its density), a common reference temperature of 59F degrees is used. Salinity is measured with a hydrometer, which is calibrated for use at a specific temperature (e.g., 75F degrees is common).
Some freshwater fish tolerate (or even prefer) a small amount of salt (it stimulates slime coat growth). Moreover, parasites do not tolerate salt at all. Thus, salt in concentrations of (up to) 1 tablespoon salt per 5 gallons water can actually help prevent and cure parasitic infections.
On the other hand, some species of fish do not tolerate ANY salt well. Scaleless fish (in general) and some Corydoras catfish are far more sensitive to salt than most freshwater fish. Add salt only if you are certain that all of your tank's inhabitants prefer it or can at least tolerate it.
Other Parameters
Ammonia
The ammonia level should ideally be zero in a mature aquarium. Fish waste, uneaten food and decaying plant matter will all contribute to the level of ammonia in the tank. However, in a mature tank, there are usually enough ammonia-converting bacteria to ensure that it never rises to detectable levels. However, there are situations which may result in a temporary rise in ammonia levels, even in a mature tank. These include:
Filter failure, or lack of maintenance
Use of medications
The addition of a large number of fish at the same time
Over-feeding
Over-enthusiastic cleaning of 'biological' filter media.
In such circumstances, the bacterial population will need time to increase or recover to cope with the demand. If fish appear unwell, testing for the presence of ammonia should be a priority.
Ammonia can cause damage at levels of only 0.1 ppm (which is below the level detected by many kits!). There may be haemorrhaging and destruction of mucus membranes, the gills are particularly likely to be damaged, and may appear reddened. As with nitrite poisoning, fish may apppear to gasp for air at the surface, and show rapid gill movement. Higher levels, of several ppm, can be fatal.
Nitrite
The nitrite level should ideally be zero in a mature tank. A temporary rise in nitrite levels may be seen for the same reasons as listed for ammonia above. However, the nitrite spike may persist longer, so if there is a delay in testing after a problem has occurred, it is more likely that nitrite will be detected. A nitrite level of only 0.1 ppm could prove harmful if exposure is prolonged. Symptoms of nitrite poisoning include gasping and rapid gill movements, which could be mistaken for a shortage of oxygen. In extreme cases, fish can actually die of suffocation because nitrite binds to the oxygen-carrying component (haemoglobin) in the blood.
Nitrate
In the past, nitrate was considered essentially harmless to fish; certainly it is far less toxic than ammonia or nitrite. It has been shown that levels of up to 1000 ppm may be required to cause death, but the effects of lower levels on long term health are not well understood. The sensitivity of different species to nitrate levels varies, and there may be long term effects on general health, growth and breeding ability.
Generally, many aquarists seem to agree that keeping nitrates below 50 ppm is necessary to prevent any long-term effects on fish health, but below 25 ppm is more desirable. Remember that many fish may come from a natural environment where there is little or no detectable nitrate. Fish which have been aquarium bred for generations are more likely to tolerate nitrates.
