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Clinical Guide to Fish Medicine


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       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.

      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.

      Carbon dioxide (CO2) is very dynamic in water as it is highly soluble, produced by animal and microbial respiration and decay processes, and used during photosynthesis. CO2 reacts with water to form carbonic acid (H2CO3) which dissociates to bicarbonate (HCO3), then carbonate (CO32−) and hydrogen ions (H+), depending on alkalinity, pH, salinity, and (to a lesser extent) temperature.

      Frequency of testing: Testing is not common and is usually done in high‐risk situations (e.g. intensive aquaculture or incoming well water).

      Sampling: CO2 should be measured on site and within about 30 minutes of sampling.

      Testing: Commercial colorimetric tests are available. CO2 can also be estimated based on pH and alkalinity using standard curves available on‐line and in Hargreaves and Brunson (1996). Because of the narrow range of pH in salt water (e.g. 7.5–8.5), a low pH in salt water may indicate high levels of CO2. A crude estimate of CO2 can be made by collecting water from the system in an open‐topped container, measuring the pH, vigorously aerating the water for about 60 minutes, and measuring the pH again. If the pH increases by more than one unit due to off‐gassing of CO2, the CO2 may be too high.

      Units: CO2 is reported in milligrams per liter (mg/L).

      Target values: Less than 20 mg/L is a common target.

      Practical considerations:

       High CO2 is problematic for fish because it affects pH and oxygen availability.

       High values are common in intensive aquaculture systems with limited off‐gassing, closed transport containers, well water, and following an algal or phytoplankton die‐off.

       Low levels are not a health concern for fish, but vascular plants, algae, and phytoplankton need CO2 for photosynthesis.

       CO2 is inversely related to pH and is a component of alkalinity, which affects pH stability.

       CO2 can vary significantly across the day because of diurnal changes in respiration and photosynthesis; values are highest at dawn (Figure A2.2).

      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.

      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