Hardness, Alkalinity and Carbon Dioxide TT76
Published on May 8, 2014

Hardness, Alkalinity and Carbon Dioxide - Tech Talk 76

What is hardness and why is it important?
You know you have hard water when soap does not lather well. Hard water causes scale deposits after it evaporates, and it forms clogging deposits in pipes and air diffusers. In aquatic systems, adequate hardness is important and depends mostly on the concentration of calcium and magnesium ions. Hardness is particularly important to newly hatched fish that obtain much of their calcium directly from the water. Generally, freshwater fish do best when hardness is maintained near 100 mg/liter @ CaCO3 (calcium carbonate).

To increase hardness, use calcium chloride (CaCl2).

Why is alkalinity important?
Alkalinity is the buffering capacity of water. That is, its ability to maintain a pH of 7.0 or above. In aquaculture, alkalinity is most often influenced by the bicarbonate ion (the negative ion in sodium bicarbonate, NaHCO3). When kept above 100 mg/liter @ CaCO3, alkalinity will usually be an effective buffer, depending on the level of carbon dioxide (CO2) in the water (see graph). Alkalinity should not fall below 80 mg/liter @ CaCO3, as it is the source of carbon for nitrifying bacteria and is consumed during nitrification. To increase alkalinity, add sodium bicarbonate.

To decrease alkalinity, add acid.

How are hardness and alkalinity related?
Hardness and alkalinity are both expressed as mg/liter @ CaCO3. In tap water, when the concentration of hardness and alkalinity are the same, both are probably due to dissolved calcium carbonate. Calcium carbonate does not dissolve well above pH 7.0, so other chemicals like calcium chloride and sodium bicarbonate can be used to adjust hardness and alkalinity independently. If alkalinity and hardness are too high, calcium carbonate will precipitate on air diffusers, heaters and other submerged equipment.

How is alkalinity affected by carbon dioxide?
Carbon dioxide may build up in water as a result of respiration by fish and bacteria. In poorly buffered water, this can cause a drop in pH below 7.0 that can inhibit nitrification. Aeration can drive off CO2 and, in the process, raise the pH. Adequate alkalinity will ensure stable pH and provide carbon for nitrifying bacteria. Carbon dioxide level can be estimated, using the graph on this page, if the pH and alkalinity are known.

Hardness, Alkalinity and Carbon Dioxide