In freshwater systems, electrical conductivity may be reported, typically in μSiemens per centimeter (μS/cm) or an older unit (μmho/cm); these units are equivalent.
Target values: The salinity of natural seawater is often 32–35 g/L (with a specific gravity of ~1.021–1.024 depending on temperature); many aquariums use a target range of 28–35 g/L. Freshwater salinity is typically <0.5 g/L or a conductivity of 60–2000 μS/cm (Table A2.2).
Practical considerations:
Inappropriate salinity or rapid changes in salinity are both problematic. Effects are more severe in stenohaline species (e.g. reef species, most pelagic sharks).Box A2.1 How to Collect a Water Sample for Later AnalysisUse a clean polyethylene or polypropylene bottle with a screw cap for most sampling (borosilicate glass can be used for pesticides).Rinse the bottle in the system with the water to be tested.Select an area of concern or an area with sufficient mixing to be representative of the system.Dunk the bottle underwater and fill completely.Cap the bottle while still under water.Dry the outside of the bottle and label with system, time, and date.Refrigerate or keep the sample on ice.Most analyses should be run within a few hours; some analyses are still reliable after freezing and thawing.
Low salinity in salt water may be seen due to freshwater contamination (e.g. rainfall, contamination from freshwater lines), insufficient salt additions, or crystallization of salts on the lids or life support system components (“salt creep”).
High salinity in freshwater or salt water may be seen due to evaporation or contamination by salts or salt water.
Rapid changes are more likely to cause problems than slow. Rapid changes are most common when fish are moved between water systems without acclimation or during freshwater or saltwater dips for ectoparasite diagnosis and treatment.
Nitrogenous Wastes (Ammonia, Nitrite, Nitrate)
Nitrogenous wastes are produced through protein metabolism by animals and decay processes (e.g. of excess food or dead plant matter). The waste products build up rapidly in closed or recirculating systems and are toxic to fish and invertebrates. The nitrogen cycle describes the conversion of ammonia into nitrite and then nitrate (Figure A2.4).
Ammonia exists as ionized ammonium (NH4+) and unionized ammonia (NH3, known as UIA). Combined, these make up total ammonia nitrogen (TAN). The unionized form (UIA) is much more toxic to fish as it can diffuse across the gills. The proportion of each depends on temperature, pH, alkalinity, salinity, and DO. More of the ammonia is in the toxic form (UIA) at high pH, high water temperature, and low salinity (e.g. a freshwater pond after a hot day). Ammonia is converted into nitrite by ammonia‐oxidizing bacteria (AOB). These include Betaproteobacteria and Gammaproteobacteria (e.g. Nitrosomonas, Nitrosospira, Nitrosolobus, Nitrosovibrio, Nitrosococcus spp.). Nitrite can be toxic to fish in freshwater systems (Lewis and Morris 1986). Nitrite is converted into nitrate by nitrite‐oxidizing bacteria (NOB). These include Nitrobacter, Nitrococcus, Nitrospina, and Nitrospira spp., among others. Nitrate is a stressor and endocrine disruptor (Hrubec et al. 1996; Camargo et al. 2005; Morris et al. 2011). These nitrifying bacteria (AOB and NOB) exist on all surfaces within the habitat, but are concentrated where surface area is highest (e.g. sand filters and undergravel filters). The nitrification process uses oxygen and alkalinity and produces carbon dioxide via bacterial respiration. Nitrate is removed from the system through water changes, algal or vascular plant use, and denitrification systems. With immature biological filtration, increases in ammonia, nitrite, and nitrate tend to follow each other (Figure A2.5).
Figure A2.4 A simple schematic of the nitrogen cycle.
Frequency of testing: Ammonia and nitrite should be assayed routinely on all systems. This may be daily on a new system, following the addition of new fish, or following an immersion treatment in order to evaluate the efficacy of the biological filtration. It may be every one to two weeks in stable systems. Ammonia should also be assayed during or following transport or restraint. Nitrate testing is often less frequent and may be done every one to two months.
Sampling: Standard sampling is described in Box A2.1. Samples for ammonia and nitrite can be stored for a few hours at room temperature, and up to 24 hours if refrigerated or 48 hours if frozen. Nitrate is more stable and samples can be stored for longer.
Testing: Commercial test kits for ammonia use one of two approaches. The Nessler method is rapid and reliable in freshwater, but is less accurate in salt water and may be falsely elevated within 24–72 hours of treatment with formalin or ammonia‐locking compounds, as these often contain formalin. The test also includes mercury which must be disposed of as hazardous waste. The ammonium salicylate method is more accurate in salt water when the modifying reagent developed by Kingsley (2014) is used. This test produces sodium nitroferricyanide; while regulations may differ, this often requires disposal as hazardous waste. The ammonium salicylate method is not affected by the presence of formalin. The test is more expensive and slightly slower. Ammonia can also be measured using ion‐specific electrodes. It is important to know water temperature, pH, and salinity to be able to assess ammonia results, since they affect the proportion of the more toxic UIA. Nitrite and nitrate tests are colorimetric (assayed by colorimeter or more accurately by spectrophotometer); commercial test kits are readily available.
Figure A2.5 Sequential increases in ammonia, nitrite, and nitrate as the nitrifying bacteria in the biological filtration mature over time.
Units: Ammonia tests report TAN in milligrams per liter (mg/L). UIA can be calculated from TAN using a conversion factor based on temperature, pH, +/− salinity. Tables are available on most test kits and conversion calculators are available on‐line (Table A2.3). If nitrite tests only report nitrite‐nitrogen in mg/L, multiply this by 3.3 to get the nitrite ion level. If nitrate tests report only nitrate‐nitrogen in mg/L, multiply this by 4.4 to get the nitrate ion level.
Target values: Targets vary depending on species, but some potential values are listed below and in Table A2.2:
UIA <0.02 mg/L.
Nitrite‐nitrogen <0.1 mg/L.
Nitrate‐nitrogen <50 mg/L for most fish and <15 mg/L for many invertebrates.
Practical considerations:
Fish health concerns are associated with high ammonia, nitrite, or nitrate, although ammonia is the most toxic of the three, and nitrite is primarily a concern in freshwater.
High ammonia, followed by high nitrite, is often seen in newly established recirculating systems in the first few weeks after animal additions, until there are sufficient nitrifying bacteria to handle the bioload. During this time, careful control of stocking density, monitoring, water changes, and other mitigation methods are essential to minimize fish exposure to the toxins while allowing the biological