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Iodine (I₂) is an essential micronutrient. It exists in water primarily as iodide (I⁻), iodate (IO₃⁻), and dissolved organic iodine. Iodide and dissolved organic iodine are considered bioavailable. Iodide and dissolved organic iodine can be oxidized to iodate, which is not available to fish. Deficiencies are associated with goiters, particularly in elasmobranchs.

Frequency of testing: Iodide and iodate should be assayed routinely in elasmobranch systems, particularly where ozone disinfection is used.

Sampling: Standard sampling is described in Box A2.1. Samples can be stored for a few days at room temperature or weeks when refrigerated or frozen.

Testing: Iodide can be assayed using high‐performance liquid chromatography (Parkinson et al. 2018). Iodide can be hard to measure accurately if nitrate levels are high.

Units: Typical units are milligrams per liter (mg/L), equivalent to parts per million (ppm). Other units are micromoles per liter (μmol/L).

Target values: In salt water with elasmobranchs, iodide should be ~0.03–0.06 mg/L (Table A2.2).

Practical considerations:

 The primary health concern is the potential for goiters in elasmobranchs due to low iodide. This is particularly common in long‐established recirculating systems with ozone disinfection, as ozone converts iodide to iodate (Sherrill et al. 2004).

 It is unknown if excess iodide in the water is a problem for fish; in other classes, excess can also lead to thyroid dysfunction and levels should likely be kept at <0.1 mg/L.

Further discussion of goiters is available in Chapter C1.