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Chlorines and chloramines can be toxic to fish and invertebrates. Chlorines (Cl₂, OCl⁻, and HOCl) should not be confused with chloride (Cl⁻); chlorines are oxidants that are highly reactive, chlorides are salts that are essential to fish. Chloramines are chlorines bound to ammonia.

Chlorines and chloramines are frequently used as disinfectants in municipal water and can be present in tap water at levels that can be toxic to fish (0.5–3.0 g/L). These must be removed from the water prior to use in fish systems. Chlorine can also come from accidental exposure to bleach or similar disinfectants. Ozone disinfection can convert chlorides (and bromides) in the water into strong oxidants and oxidative by‐products. These should be confined to the ozone contact chamber and not released into the fish habitat.

“Free chlorine” is the sum of the active oxidants (Cl₂, OCl⁻, and HOCl). “Total chlorine” is the sum of free chlorine and combined chlorine (e.g. chloramines). There is no direct test for chloramines.

Frequency of testing: Chlorines should be assayed where they are considered a risk. Incoming municipal freshwater should be tested before and after carbon filtration or other treatment to ensure chlorines have been removed. With ozone disinfection, system water should be tested for residual oxidants, including free and total chlorine (and free and total bromine). Regular testing is more useful than individual results, because of the error margin inherent in many of the test methods.

Sampling: Standard sampling is described in Box A2.1. It is particularly important that the bottles are properly cleaned prior to use; any tap‐water contamination will artificially increase the results.

Testing: Free and total chlorines are measured by colorimetry (based on the amine DPD) or potentiometry. Handheld meters are available (e.g. Pocket Colorimeter™ II, Chlorine, Hach, Loveland, CO). The error margin for many of these tests is ±0.02 mg/L. Gel standards that simulate specific chlorine values can be used for a quick check of instrument performance and accuracy, but they cannot be used to calibrate instruments or assess technique (e.g. contamination of bottles).

Units: Free and total chlorines are reported in milligrams per liter (mg/L).

Target values: Total chlorines should be <0.03 mg/L for most fish and <0.01 mg/L for sensitive species (Table A2.2).

Practical considerations:

 The main health concern is high levels of chlorines or chloramines, as they are toxic to fish.

 High levels are most often from untreated municipal water or failure of dechlorination methods (e.g. exhausted activated carbon, miscalculation of thiosulfate to inactivate chlorine).

 Management typically consists of water changes, aeration, the addition of activated carbon filters, or treatment of the water with sodium thiosulfate, ultraviolet light, or binding products such as AmQuel® and AmQuel Plus™ (Kordon LLC) and AMMO‐LOCK® (API).

 Low levels are not a concern.

Further discussion of chlorine and chloramine toxicity is available in Chapter C1.