Water Chemistry 101

Water Chemistry 101

Understanding the key elements that make up your water quality is an important part in advancing from beginner to experienced aquarist.  There is an old saying , we aren’t actually managing the fish, we are managing the water.  Here is a basic rundown of what we measure and why.

PH (Potential Hydrogen)

What is It ?

PH is a measure of the acidity of water, the scale runs from 0 to 14  with PH of 7.0 being neutral, below 7.0 being acid and above 7.0 being alkaline or base.


KH  (Carbonate Hardness CO3)

What is it ?

Carbonate Hardness is the amount of Carbonates (CO3) in the water, Carbonates directly effect PH. As dKH increases so does PH as dKH decreases so does PH.  You will sometimes see KH referred to as Alkalinity but this isn’t strictly correct, Alkalinity is the measure of all bases in the water not just the carbonates. 
Just to confuse matters there are two common scales used to express Carbonate levels.  Non fish people tend to measure CO3 in parts per million of Calcium Carbonate (ppm/CACO3) but most aquarium tests give a result on a scale known as dKH or Degrees Carbonate Hardness. 
1dKH = approx 17.9 PPM CACO3.
The minimum level you should aim for is at least 3dKH (55ppm CACO3).  Most typical community fish are happy with KH from 3 to around 6, African Cichlids prefer levels of 8+ (Which is why most African Chiclid Tanks have PH above 8.0)


Why Does it matter ?

Low KH can lead to what is known as a PH crash (Which is more correctly a KH crash)  Because KH determines PH, if the aquarium gets low in Carbonates (or runs out all together) then the PH can suddenly plunge to below 4.0 in a matter of hours, this is invariably fatal to most fish.   Carbonates are used up in 2 main ways…  Biological activity in the filtration can use up Carbonates, and some aquatic plants convert Carbonates (CO3) to Carbon Dioxide (CO2)  which depletes Carbonate levels.  Fortunately its very easy to replace lost Carbonates by adding plain Baking Soda  NAHCO3.  Although prolonged use of Baking Soda can lead to elevated levels of Sodium.

How do we measure KH ?

With a liquid test kit, I prefer the JBL kit but API do a good one as well.  

GH (General Hardness)

What is it ?

Basically GH is the dissolved mineral content of the water, specifically calcium and magnesium but also manganese, strontium, copper, potassium etc etc. 




TDS / Conductivity