laevis and X. tropicalis, as examples (Fisher 2000; Koerber and Kalishman 2009; Schultz and Dawson 2003).
The development and maintenance of the biofilter is critical for limiting ammonia and nitrite accumulation in recirculating systems. The biofilter must be of sufficient size (i.e., contain a sufficient quantity of bacteria) to be capable of processing the bioload (level of nitrogenous waste) entering the system. The microorganisms supported by the biofilter require certain water quality parameters. Alterations in the aquatic environment (e.g., rapid changes in salinity, temperature, and pH) as well as the addition of chemicals or antimicrobials may significantly affect the microbial ecology of the system and therefore water quality and animal well-being. If damaged, biofilter recovery may take weeks (Fisher 2000). Changes in water quality parameters (e.g., pH, ammonia, and nitrite) may negatively affect animal health and the efficiency of the biofilter, so species sensitive to change in water quality outside of a narrow range require more frequent monitoring.
Continuous or timed flow-through systems can be used where suitable water is available to support the species to be housed (e.g., in aquaculture facilities). These systems may use extremely large volumes of water as it is not reused. The water may be used “as is” or processed before use, for example by removing sediments, excessive dissolved gases, chlorine, or chloramines, and by disinfecting with UV or ozone (Fisher 2000; Overstreet et al. 2000). Static systems vary in size from small tanks to large inground ponds, and may use mechanical devices to move and aerate water.
The general concepts discussed in the Terrestrial Animals section also apply to the aquatic setting. Most aquatic or semiaquatic species (fish, amphibians, and reptiles) used in research are poikilotherms, which depend, for the most part, on the temperature of their environment to sustain physiologic processes, such as metabolism, reproduction, and feeding behavior (Browne and Edwards 2003; Fraile et al. 1989; Maniero and Carey 1997; Pough 1991). Temperature requirements are based on the natural history of the species and can vary depending on life stage (Green 2002; Pough 1991; Schultz and Dawson 2003). Water temperature may be controlled at its source, within the life support system, or by controlling the macroenvironment. Some semiopen systems (e.g., raceways by a river) depend on source water temperature and thus enclosure water temperature will vary with that of the source water.
The volume of water contained in a room can affect room temperature, temperature stability, and relative humidity. Likewise the thermal load produced by chiller/heater systems can affect the stability of the macroenvironmental temperature. Air handling systems need to be designed to com-