General hardness and carbonate hardness 

All fresh water sources contain calcium and magnesium salts in varying quantities, general hardness (gH) is a measure of the amount. Water high in calcium salts is 'hard' and water low in dissolved salts is 'soft'.


Levels of hardness
Very hard Over 28ºdGH
Hard 20-28ºdGH
Medium hard 15-19ºdGH
Medium 10-14ºdGH
Soft 6-9ºdGH
Very soft 0-5ºdGH

The majority of fish offered for sale are best suited to neutral or soft/acidic water. There are some species that thrive best in harder more alkaline water conditions such as African Rift Lake Cichlids.  

The carbonate hardness (kH) of water is an important measurement because the level of carbonate hardness is the basis for the pH level in a given water sample. A strong carbonate hardness reserve (buffering capacity) in water means a very stable pH. The buffering capacity can be kept stable with the help of regular water changes but will slowly drop over time due to organic processes. 

Luckily the range of products available on the market today are designed for ease of use. Adding the prescribed dose to the aquarium will increase buffering capacity and some will even adjust the pH too. These 'buffers' are added to any fresh water introduced to maintain the desired levels.

Back to water chemistry or on to Nitrate (NO3)

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