Iodide and Iodate
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.
Heavy Metals
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.
Turbidity/Suspended Solids
Turbidity and total suspended solids (TSS) are a measure of the suspended material in the water column, which can interfere with respiration and light penetration.
Frequency of testing: Turbidity may be tested routinely where indicated (e.g. to assess effectiveness of mechanical filtration) or in response to an upcoming treatment (e.g. copper sulfate immersion).
Sampling: Testing is usually done on site in the affected system.
Testing: In ponds and other slow‐moving surface water, a black and white Secchi disk can be lowered into the water to record the depth at which the pattern cannot be seen (typically based on an average of two readings). Nephelometric tests measure the scatter from a focused light beam. Handheld probes are available.
Units: Secchi depth is reported in centimeters or inches; a low value is equivalent to high turbidity. Nephelometric tests are reported in nephelometric turbidity units (NTU), Formazin turbidity units, or a variety of other units depending on the meter.
Target values: Target for ponds may be Secchi depth of >50 cm (18 in) or turbidity < 1‐10 NTU.
Practical considerations:
High turbidity can be an issue for fish because it can affect gill function, dissolved oxygen, feeding, fecundity, light penetration, and aesthetics. The particulates can also cause clogging of fine mechanical filters.
High turbidity may be due to phytoplankton or bacterial blooms, insufficient mechanical filtration, disturbance of the substrate, or contamination (e.g. runoff).
Low turbidity may be a concern for species that have evolved in brown or black pond or river systems.
Microbiome and Bacterial Testing
Aquatic systems are rich in microbes, including bacteria, archaea, protists, fungi, and microalgae such as diatoms. This microbiota is found within the water column and attached to all surfaces. On wet surfaces, this microbiota creates a sheltered scaffold known as the biofilm. Assessment of this microbiome in aquarium systems is in its infancy, including the relationship to fish health and biological filtration.
Bacterial indicator tests use selective bacterial media designed to look for point sources of human fecal contamination in surface water. Various commercial tests are available (e.g. total coliforms and fecal coliforms on Colilert, or Enterococcus spp. on Enterolert, both by Idexx, Westbrook, ME). In‐house testing relies on either membrane filtration or multiple‐tube fermentation methods with results available one to two days later as number of colony‐forming units (CFU)/100 mL for membrane filtration and most probable number (MPN)/100 mL for multiple‐tube fermentation; these results are equivalent. In the United States, targets are often extrapolated from state or US‐EPA regulations for swimming beaches or USDA regulations for marine mammal habitats. These tests likely have value when assessing open systems with human access where point contamination with human sewage is possible (e.g. swim with ray programs in outdoor lagoons). However, these bacterial indicator tests are problematic in most fish systems because of a high risk of false positives (e.g. growth of plant‐based Enterococcus spp. or nonselected genera such as Vibrio spp.) and because they may not have any relation to pathogen load (Pisciotta et al. 2002; Culpepper et al. 2016). Rather than a measure of potential fish morbidity, these tests may be most useful in determining if disinfection needs