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Heavy metals are potentially toxic to fish. They are particularly common where surface or ground water is used. Potential toxins include copper, zinc, lead, mercury, manganese, and arsenic.

Frequency of testing: Heavy metals are usually tested based on a risk analysis. For example, the well‐water source for a pond may be assayed routinely for iron, while a tropical saltwater system on treatment with copper sulfate should be assayed daily for copper ions.

Sampling: Standard sampling is described in Box A2.1. Samples can be stored for weeks or months if frozen.

Testing: Commercial colorimetric tests are available for some heavy metals (e.g. copper, iron); accuracy varies and results are best assessed using a spectrophotometer. Most other tests for heavy metals rely on atomic absorption spectrophotometry.

Units: Heavy metal concentrations are typically reported in milligrams per liter (mg/L), equivalent to parts per million (ppm), or in micrograms per liter (μg/L), equivalent to parts per billion (ppb). It is important to differentiate between the free/ionic concentration (the more toxic form) and total concentration (free and combined or chelated).

Target values: This can be hard to determine, because toxicity depends on factors such as species, life stage, pH, hardness, alkalinity, temperature, and the presence of other stressors. In general, values should be <0.03 mg/L for most heavy metals.

Practical considerations:

 While some metals are considered essential ions, the main health concern is high levels of heavy metals, particularly in the free/ionic form.

 Common causes are copper sulfate treatment, contaminated source water, drip contamination of the system, or leaching from décor, substrate, enclosure walls, or water lines.

 Toxicity is more likely at low pH, hardness, and alkalinity, and at high water temperature.

Further discussion of heavy metal toxicity is available in Chapter C1.