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Understanding water hardness

Water hardness is a measure of the total dissolved salt content in water or the quantity of certain metallic ions. e.g. calcium and magnesium. The general hardness (GH) refers to the total content of these combinations of salts and can be divided into carbonate hardness (KH) and permanent hardness. KH can be removed through boiling the water, whereas permanent hardness remains after boiling.They’re often unavoidably introduced in new plants and blend into the aquarium environment.

There is no universal, ideal water hardness. The perfect conditions are those, closest to your fish’s natural environment.

A high KH plays a significant role in the aquarium by keeping the pH stable. When fish breathe and your filter converts toxic waste this releases hydrogen ions which can drop the pH. A high KH will soak these ions up like a sponge and stop the pH from changing too much, causing stress to your fish. If you are keeping fish from the Amazon then you will want to have a lower water hardness but if you are keeping African cichlids or pond fish then a higher hardness would be better as this is what they are naturally used to. Water hardness can be raised by adding a calcium based substrate or by adding a water buffering product. To lower water hardness levels perform small water changes with reverse osmosis or rain water.

Increased water hardness is caused by:

  • Where your tap water is sourced from – test the water coming out of your tap to see if it is hard or soft.
  • Calcium substrates like coral sand.
  • Some ornaments or pond walls which leach chemicals into the water.

Decreased water hardness is caused by:

  • Where your tap water is sourced from
  • Fish respiration and biological filtration over an extended period of time – Old tank syndrome
  • Plant growth
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