Most freshwater fish live within a pH range of 5.5 to 7.5 (African cichlids 7.5 to 8.4).

Most Saltwater species are happy around 8.4
For saltwater specimens there is less margin for error than in freshwater aquaria. Keep Your marine aquarium buffered!

In ALL cases, it is the rapid fluctuation that is the most dangerous!

In general. most aquaria will "slide" as ph is reduced by organic decay.  (water changes and vacuuming gravel
are vital and the simplest means to correct for this decrease.)
Excess waste product produces carbonic acid, which acidifies the water and lowers the pH.

Since the scale is logarithmic, this range represents a variation of over a 1000 times. 
Even an apparently small change in pH can affect fish, causing stress or death.

There are also indirect consequences that can affect fish.
 
Changes in pH will affect the toxicity of many dissolved compounds.
For example, ammonia becomes more toxic as pH increases.
Also consider that medications are affected which increases the possibility of overdosing in Lowered ph conditions.

Nitrifying bacteria, essential in the conversion of ammonia to nitrate also have a pH range preference,
which is between 7.5 and 8.6.
 

Methods of altering the pH in your freshwater aquarium:

lower pH (freshwater)
· Use a commercial acid buffer
· Filtering water over peat
· Add bogwood to the tank
· Inject carbon dioxide CO2
· Water changes with softened water or RO (Reverse Osmosis) water

to raise the pH
· Aerate the water, driving off the carbon dioxide (CO2)
· Filter over coral or limestone
· Add rocks containing limestone to the tank or use a coral sand substrate
· Use a commercial alkaline buffer

In Saltwater

· Water changes
. Buffers

Remember: Any changes to pH should be done gradually if possible!!!

Water that is poorly buffered (low kH or temporary hardness) will be subject to higher pH fluctuations
than well-buffered water.  As a general rule, hard water is usually alkaline (above 7) and well buffered,
 whereas soft water (below 7) is usually slightly acidic and poorly buffered.
 Permanent hardness also has an affect on the pH therefore in order to completely understand pH,
we must also understand water hardness.

Water hardness

Water accumulates many dissolved substances before it reaches our taps.
 Hardness is a measurement of the concentration of metal ions such as calcium, magnesium, iron, and zinc.
 Most of these concentrations are acquired as rain water passes over rocks.  In most water it consist mainly of
calcium and magnesium salts, with trace amounts of other metals.

There are two types of hardness that we need to consider.

 Permanent hardness and alkalinity (kH).(often referred to as carbonate or temporary hardness) 
  The sum of both types of hardness is called general hardness (gH)

Alkalinity or temporary (carbonate) hardness (kH) refers to the hardness derived mainly from
carbonate and bicarbonate ions and directly reflects the buffering capacity of the water.
  It can be precipitated and removed by boiling the water.  This is why lime-scale forms in kettles and shower heads.

Permanent hardness measures ions such as nitrates, sulphates, and chlorides etc, and cannot be removed by boiling.
While there is a connection between water hardness and buffering, hardness is a product of mainly calcium and
 magnesium ions and buffering is produced by bicarbonate and carbonate ions.
  As mentioned earlier, hard water is usually well buffered and soft water is usually less buffered.
 It is possible though, based on different water compositions, to have hard water that is poorly buffered
 or soft water that is well buffered.
  The way to establish the makeup of your local water is by using a test kit and test for
 both gH (general hardness) and kH (temporary hardness).kH

Carbonate hardness or temporary hardness.
 Measures the buffering capacity or the ability to absorb and neutralize added acid without major changes to pH.
 Think of buffering capacity as a big sponge, the higher the buffering, the bigger the sponge. 
How much buffering does your tank need?  The higher the kH (the bigger the sponge),
 the more resistant to pH changes your water will be.
 A tank's kH should be high enough to prevent large pH swings over time.
 If your kH is below roughly 4.5 OdH, you should pay special attention to your tank's pH
 (e.g., testing periodically) until you get a feel for how stable the pH is.

Buffering is both good and bad.
 On the good side, the nitrogen cycle in our tanks produces nitric acid (nitrate).
 If we don’t have buffering (kH), the pH will drop over time. 
Sufficient buffering will keep the Ph stable.
 On the bad side, hard water almost always has a large buffering capacity and if the pH is to high for your fish,
 this large buffering capacity will make it more difficult to lower the pH.

Buffering is sometimes referred to as "alkalinity" but should not be confused with "alkaline".
 Alkalinity refers to buffering and alkaline refers to a solution that is base rather than acid (pH).

Aquariums with a low kH will require more attention to water changes to control the nitrate level
reducing the tendency for the pH to drop.

As with pH, there are ways to increase and decrease the buffering capacity of your water.

Ways to increase kH:

· Adding commercially available products to increase buffering capacity
· Adding sodium bicarbonate (baking soda).
 One teaspoon of baking soda added to 50 liters of water can raise the kH of the water
 by approx 4 OdH without a major affect on pH.
· Adding an air stone to increase surface turbulence driving off carbon dioxide (CO2)

Ways to lower kH

· Adding commercially available products to decrease the buffering capacity.
· Injecting carbon dioxide (CO2)
· Use reverse osmosis (RO) water.  You can mix tap water with reverse osmosis water to achieve the desired kH.

It is not a good idea to use distilled water in your tank.
 By definition, distilled water has essentially no kH.
 That means that adding even a little bit of acid will change the pH significantly (stressing fish).
 Because of its instability, distilled (or any essentially pure water) is never used directly.
 Tap water or other salts must first be added in order to increase its gH and kH.

gH

General hardness (GH) refers to the dissolved concentration primarily of magnesium and calcium ions.
 Other ions can contribute to water hardness but are usually insignificant and difficult to measure.
 When fish are said to prefer ``soft'' or ``hard'' water, it is gH, not kH that is being referred to.
 gH will not directly affect pH although "hard" water is generally alkaline due to some interaction of gH and kH.

Incorrect gH will affect the transfer of nutrients and waste products through cell membranes
 and can affect egg fertility, proper functioning of internal organs such as kidneys and growth.
 
 Within reason, most fish and plants can successfully adapt to local gH conditions, although breeding may be impaired.

Some test kits measure gH or general hardness in German degrees hardness or OdH,
which is equal to 17.9 mg/L.  Since mg/L is equal to ppm (parts per million)
simply multiply the degrees OdH times 17.9 if you prefer to work with ppm.

 The following table will give an idea of how hard your water may be after reading the test results.

General Hardness Table

0 to 4 dH 0 to 70 ppm                 Very Soft 
4 to 8 dH  70 to 140 ppm             Soft 
8 to 12 dH  140 to 210 ppm         Medium Hard 
12 to 18 dH  210 to 320 ppm       Fairly Hard 
18 to 30 dH  320 to 530 ppm       Hard 
Higher                                          Very Hard 

Ways to increase gH

Adding limestone to the aquarium (this will also increase kH which in turn will increase pH)
Adding calcium carbonate will raise gH and kH

Ways to reduce gH

Adding peat moss to your filter
Use commercially available water softening pillows or a water softener
Mixing tap water with reverse osmosis (RO) water.

It is more difficult to change gH without affecting kH than it is to change kH without affect gH as you can see.

Conclusions

While distinct, pH, kH and gH interact and affect each other.  If you change one parameter,
 be sure and monitor the others to see the affect.

Make changes gradually.

When making changes it is usually best to do it in containers outside the aquarium,
then add the treated water to the aquarium.

If you have a low kH, increase water changes accordingly and monitor pH more frequently.

Understand that decorations such as driftwood, bogwood, limestone, filtering with peat, etc.
will affect the kH and pH of the aquarium.

Saltwater

Salinity and Specific Gravity

As a general rule most saltwater systems are best kept at a specific gravity 1.020 - 1.025
To discourage parasites - lower specific gravity to 1.018

The preferred water for marine and reef tanks is RO/DI water, as it is in a very pure form.
The commercial salt mix contains all the salts and minerals needed.

Salinity describes the content of these dissolved salts and minerals
 in the water and is measured in parts per thousand (ppt).

The easiest and most common instrument to measure the salt concentration in aquaria is the hydrometer.
A hydrometer is a device that measures the density or specific gravity of a liquid.
 The more salt in the water the more dense it is.

It is important to understand that salinity and specific gravity are related - but not the same.
Salinity can be measured by (a) boiling down a water sample, (b) measuring conductivity by electronic means,
and (c) other laboratory methods which are either too complex or too expensive.

Specific gravity indicates density, while salinity refers to the actual weight of the salt.

The hydrometer works on the principle that a solid body displaces its own weight of the liquid in which it floats.
 The hydrometer is calibrated at 60˚F (15.55˚C) in which distilled water equals 1.000 as the initial point.
The readings will rise with increasing density of the water.

As the instrument is calibrated at a temperature of 60˚F (15.55˚C),
it requires that the water to be tested will also have a temperature of 60˚F (15.55˚C) in order to get an accurate reading.

Water will expand or contract if temperatures vary; therefore the density fluctuates with temperature as well.
The chart below shows the actual density in relation to the water temperature.

Actual Specific Gravity

Example:

A marine tank maintained at a temperature of 82˚F (27.77˚C) with a hydrometer reading of 1.021
translates into an actual specific gravity of 1.0237

Salinity in ppt

Example:

A marine tank maintained at 80 F and a specific gravity reading of 1.022 has a salinity content of 31.8 ppt

Stability in specific gravity is an important factor. Any fluctuation will trigger some effects.

Only freshwater will evaporate from the tank leaving the salt and minerals behind.
 Therefore only freshwater should be used to top off the aquarium.

Further, the water level is also of importance, as more freshwater evaporates
the density of  the remaining water will increase, thus raising the salinity.

A new batch of salt mix is only required with water changes.

Measurements with the hydrometer should be taken below the surface,
 avoiding air bubbles as not to influence the reading.

The hydrometer should be rinsed with freshwater after use to avoid any residues.

Ammonia -  Nitite - Nitrate

The Nitrogen cycle

Here  the Nitrogen cycle is explained