Sunday,26 September 2021, 7 : 23 AM

Water Chemistry in Aquariums

Symphysodon spp.:Discus
Image Credit: Hayath M


We all know that fish live in water, and its composition is vital for good health of fish.
The parameters of water in our aquarium depends on quality of water that we get and various things that are dissolved in water.
Water Chemistry is an important, but often a baffling aspect of Fish keeping.

We will discuss three important aspects of Water Parameters,

  • Ph
  • Gh (General Hardness)
  • Kh(Carbonate hardness).

Before diving in detailed discussion of water chemistry let us understand the Chemical composition of Water

Chemical Composition of Water

Two elements H (Hydrogen) & O (Oxygen) combine to form water.


Diagrammatic Representation of Water Molecule
Image Credit: Wikipedia

Two atoms of Hydrogen combine with one atom of Oxygen to form water, hence the Chemical formula of water is H2O.
However each molecule of water exists in ionized state, as H+(Hydrogen ions) and OH- (hydroxyl ions).
These ions are responsible for nearly all chemical properties of water.

Now we are going are to discuss the three important parameters of water.

  • Ph

    As stated above water exists as ions and is referred as H+ and OH- .
    Pure water contains nearly identical number of H+ & OH- ions and is said to be neutral.
    However the number of two ions is not exactly equal.
    There is a diff of one in ten million or 10-(7) between the two ions in pure water.
    This is also represented as Log10[10-7].
    Hence we say that pure water has ph of "7".

    Now as we dissolve chemicals in water , the chemical reactions change the ratio of these ions, hence the water gets either acidic or alkaline.
    If we dissolve say Nitric acid(HNo3) in water, the acid decomposes in H+ and No3-.
    This increases the number of H+ ions and ph drops. The reverse takes place when an alkali is dissolved in water.

    An important point to note here is that ph scale is logthermic in nature
    , so there is diff of 10 between ph of 7-8 , but a 100 times diff between ph of 7-9.

  • General Hardness (GH)

    General hardness refers to mainly number of Calcium (Ca++ ) and magnesium (Mg ++ ) ions in water.
    It is commonly measured in equivalent terms of Calcium Carbonate (CaCo3) in ppm (parts per million) or German degree (dh). One dh= 17.8 ppm of (CaCo3).
    Pure water has gh=0

    Some guidelines of water hardness are :

    dh ppm Hardness of water
    00 - 04 00 - 70 Very soft
    04 - 08 70 - 140 Soft
    08 - 12 140 - 210 Medium hard
    12 - 18 210 - 320 Fairly hard
    18 - 30 320 - 530 Hard

    When we talk about soft water fish or hard water fish it is the GH that is highly relevant.


Carbonate hardness refers to amount of negatively charged ions of bicarbonates (HCO3-) and carbonates (CO3-) in the water.
The amount of carbonates is very important it stabilizes the ph of water.
This is called buffering capacity of aquariums.
Aquariums with very low kh are prone to large fluctuations in ph.
This is primary reason that pure RO water is never good for aquariums as it has no buffering capacity and ph swings are high.

Water parameters & Fish:

All Fish have a bodily function called Osmo regulation.
In simpler terms it is the ability of fish to maintain water and mineral contents in its body by either taking in or expelling them.
This process takes place throughout the body of fish.
Water goes in through the mouth, exists via gills as well as skin, during this process minerals are either absorbed or expelled by fish.
Now if this exchange is disrupted there will be either an excess or deficiency of some mineral.
In humans and higher animals the same function is performed by absorption from food.
However in the case of fish these minerals are easily available in water all along them, hence they use this process.


Osmo regulation in Fish
Image Credit: Goggle Images

Now over millions of years of development fish have adapted to either absorb or expel minerals in a given time depending upon the environment where they live.
Hard water and soft water fish have different absorption rates for minerals.
Now fish can and do adept to the water they are put in, but in the long run it is better to give them the parameters they have adapted in their native waters.
A very important point to note here is that fish require stable water parameters,
If the water parameters go on changing the fish will try to regulate their osmo regulation, this will put a strain on their systems.