High nitrate is often seen in long‐established recirculating systems without denitrification or adequate water changes or following contamination of a system (e.g. with nitrogenous fertilizer).Table A2.3 Fraction of the total ammonia nitrogen (TAN) that is present as unionized ammonia (UIA) at various temperature–pH combinations. Multiply TAN (mg/L) by this conversion factor to obtain UIA (mg/L).Source: Modified from Emerson et al. (1975), reprinted with permission. © John Wiley & Sons.TemperaturepHCelsiusFahrenheit6.06.57.07.58.08.59.00320.00010.00030.00080.00260.00820.02550.07641340.00010.00030.00090.00280.00890.02770.08252360.00010.00030.00100.00310.00970.03000.08903370.00010.00030.00110.00340.01050.03250.09604390.00010.00040.00120.00360.01140.03520.1035410.00010.00040.00130.00400.01230.03800.1116420.00010.00040.00140.00430.01340.04110.1197450.00010.00050.00150.00460.01450.04440.1288460.00020.00050.00160.00500.01570.04790.1379480.00020.00050.00170.00540.01690.05160.14710500.00020.00060.00190.00590.01830.05560.15711520.00020.00060.00200.00630.01970.05990.16812540.00020.00070.00220.00680.02130.06440.17913550.00020.00070.00240.00740.02300.06920.19014570.00030.00080.00250.00800.02480.07430.20215590.00030.00090.00270.00860.02670.07970.21516610.00030.00090.00290.00930.02870.08540.22817630.00030.00100.00320.01000.03080.09140.24118640.00030.00110.00340.01070.03310.09780.25519660.00040.00120.00370.01150.03560.1050.27020680.00040.00130.00400.01240.03820.1120.28421700.00040.00140.00430.01330.04100.1190.29922720.00050.00150.00460.01430.04390.1270.31523730.00050.00160.00490.01540.04700.1350.33024750.00050.00170.00530.01650.05030.1440.34625770.00060.00180.00570.01770.05380.1530.36326790.00060.00190.00610.01890.05750.1620.37927810.00070.00210.00650.02030.06150.1720.39628820.00070.00220.00700.02170.06560.1820.41229840.00080.00240.00750.02320.07000.1920.42930860.00080.00250.00800.02480.07460.2030.446Values for °F are rounded off to the closest integer.
Increases are more likely to be problematic in warm water and freshwater systems because more of the ammonia is in the unionized, toxic form.
Low levels are never a concern for fish, although vascular plants and algae need nitrate for healthy growth.
Further discussions of ammonia, nitrite, and nitrate toxicity are available in Chapter C1.
pH
Water pH represents the concentration of hydrogen ions (H+) on a logarithmic scale. Because it is a logarithmic scale, a change in pH from 8 to 7 represents a 10‐fold increase in [H+] and from 8 to 6 represents a 100‐fold increase; small changes in pH represent large changes in [H+]. Water pH affects behavior, physiology, disease dynamics, drug pharmacokinetics, and sensitivity to toxins.
Frequency of testing: Assays of pH should be performed routinely; this often consists of testing the incoming water regularly or with each production batch, and testing stable systems every two weeks. Testing frequency may be increased if there is a high risk of pH fluctuations (e.g. a low alkalinity system). pH should also be assayed in transport water and prior to the addition of fish to buffered tricaine methanesulfonate (MS‐222).
Sampling: Standard sampling is described in Box A2.1. Samples should be assayed within a few hours; samples cannot be frozen and assayed later.
Testing: Colorimetric tests are common, but often have an accuracy of ±0.5 (Baird et al. 2017). Handheld pH meters that use electrometric or potentiometric methods are more accurate and readily available; they must be calibrated prior to each use.
Units: The scale ranges from 0 to 14, with no units; a pH of 7 is neutral, lower values are acidic and higher values are alkaline.
Target values: In freshwater systems, target pH may be between 5.5 and 7.5. In saltwater systems, target pH may be between 7.5 and 8.5 (Table A2.2). Some species have specific needs, e.g. African Rift Lake cichlids (Cichlidae) prefer high pH, while discus (Symphysodon spp.) and freshwater angelfish (Pterophyllum spp.) prefer low pH.
Practical considerations:
pH needs to be within the target range for all animals in the system. While preferences are similar for most marine species, preferences of freshwater species vary widely. For example, many commonly available tropical freshwater fish are kept together at a neutral pH but prefer high pH (e.g. guppies, Poecilia reticulata) or low pH (e.g. neon tetras, Paracheirodon innesi). If a specific ecology has been targeted (e.g. Lake Victoria or Rio Negro), the life support system should be designed to replicate that pH.
pH changes in either direction can be problematic to animals and nitrifying bacteria.
Problems are often due to changes in buffers (such as carbonates and bicarbonates), carbon dioxide or other acids in the systems, gas exchange in the system, or the pH of the source water. New concrete can also leach materials that affect pH.
Rapid changes are more likely to cause problems than slow changes and are more likely with low alkalinity.
If pH increases in a system with ammonia or heavy metals, the proportion of the toxic forms increases and morbidity and mortality become more likely. This scenario is most clinically relevant with closed transport containers: when closed plastic bags are opened post‐transport, the accumulated CO2 escapes, water pH increases, and the high levels of ammonia in the transport water become more toxic. Acclimation (the slow addition of the new water) should start as soon as possible after opening closed containers.
pH can vary significantly across the day because of diurnal changes in animal and plant respiration and photosynthesis, particularly in heavily planted systems like many outdoor ponds. pH values are lowest at dawn (Figure A2.2).
Further discussion of pH stress is available in Chapter C1.
Alkalinity and Hardness
Alkalinity is the concentration of anions or bases in water (particularly bicarbonate [HCO3−] and carbonate [CO32−], but also hydroxide [OH−], borate [BO33−], and phosphate [PO43−]). These act as buffers and alkalinity is a measure of the buffering capacity of the water. High alkalinity helps to limit rapid changes in pH. Hardness is the concentration of divalent cations (particularly calcium [Ca2+] and magnesium [Mg2+], but also strontium [Sr2+], ferrous iron [Fe2+], and manganese [Mn2+]). These are best considered essential cations that support biological processes in fish and invertebrates (particularly corals). High alkalinity is often associated with high hardness and high pH, but that relationship does not always hold (e.g. sodium bicarbonate contributes to alkalinity but not hardness). The relationship is complex; Boyd et al. (2016) provides a good summary of the inter‐relationships.
Frequency of testing: Alkalinity and hardness should be assayed routinely; this may involve testing the system every two to four weeks. Alkalinity should be assayed prior to copper sulfate immersion treatment, since low alkalinity increases the risk of toxicity. Hardness should be closely monitored in rearing systems as inappropriate hardness can affect egg hatchability and fry survival.
Sampling: Samples can be stored for a few days at room temperature or a few weeks when refrigerated.
Testing: Commercial colorimetric test kits are available. EDTA titration and atomic absorption spectrometry tests are available. Spectrophotometric testing for specific anions or cations may be considered in some systems, e.g.:
Calcium