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Alkalinity is the concentration of anions or bases in water (particularly bicarbonate [HCO₃⁻] and carbonate [CO₃²⁻], but also hydroxide [OH⁻], borate [BO₃³⁻], and phosphate [PO₄³⁻]). These act as buffers and alkalinity is a measure of the buffering capacity of the water. High alkalinity helps to limit rapid changes in pH. Hardness is the concentration of divalent cations (particularly calcium [Ca²⁺] and magnesium [Mg²⁺], but also strontium [Sr²⁺], ferrous iron [Fe²⁺], and manganese [Mn²⁺]). These are best considered essential cations that support biological processes in fish and invertebrates (particularly corals). High alkalinity is often associated with high hardness and high pH, but that relationship does not always hold (e.g. sodium bicarbonate contributes to alkalinity but not hardness). The relationship is complex; Boyd et al. (2016) provides a good summary of the inter‐relationships.

Frequency of testing: Alkalinity and hardness should be assayed routinely; this may involve testing the system every two to four weeks. Alkalinity should be assayed prior to copper sulfate immersion treatment, since low alkalinity increases the risk of toxicity. Hardness should be closely monitored in rearing systems as inappropriate hardness can affect egg hatchability and fry survival.

Sampling: Samples can be stored for a few days at room temperature or a few weeks when refrigerated.

Testing: Commercial colorimetric test kits are available. EDTA titration and atomic absorption spectrometry tests are available. Spectrophotometric testing for specific anions or cations may be considered in some systems, e.g.:

 Calcium in hard coral and egg‐ or larval‐rearing systems.

 Ferrous iron in well water.

 Phosphates in hard coral systems, outdoor ponds where increases (e.g. from fertilizer) can cause algal blooms, and long‐established closed systems where phosphates from food items build up over time.

Units: In the United States, hardness and alkalinity are reported as milligrams per liter (mg/L) of CaCO₃, often reported as just mg/L. They can also be reported as milliequivalents per liter (mEq/L) of CaCO₃. Conversion of mg/L and mEq/L depends on the molecular weight and ionic charge. In the case of CaCO₃, multiply the mEq/L value by 100 to get mg/L of CaCO₃. Hardness may also be reported as the German or French degree of hardness (dH or °f respectively).

Target values: Freshwater may be soft (hardness 40–75 mg/L or 2–4 dH) or hard (hardness 75–150 mg/L or 4–8 dH) and with an alkalinity of 50–200 mg/L. Salt water usually has a high hardness (150–300 mg/L or 8–16 dH) and high alkalinity (>200 mg/L) (Table A2.2). A possible target range for free calcium is 25–100 mg/L.

Practical considerations:

 Alkalinity and hardness need to be within target ranges for all animals in the system.

 The most common problem is a gradual decrease in alkalinity (and to a lesser extent hardness) in closed systems as these ions are used by the animals. When managing closed systems, buffers and salts are routinely added to compensate for this decline.

 Alkalinity and/or hardness are also affected by the addition or removal of salts, e.g. oyster shell (CaCO3), calcitic limestone (CaCO3), dolomitic limestone (MgCO3 and CaCO3), hydrated/slaked lime/limewater (Ca(OH)2), sodium bicarbonate (NaHCO3), sodium carbonate/soda ash (Na2CO3), and calcium chloride (CaCl2). These salts may be part of the substrate (e.g. crushed coral), décor, or enclosure walls (e.g. concrete).

 Low alkalinity increases the risk of rapid changes in pH, and low alkalinity and hardness increase the dissolution and absorption of heavy metals. Copper sulfate treatment is contraindicated in low alkalinity water for these reasons.