steady supply and the health and suitability of the organism as a food. Care should be taken to feed a complete diet to avoid nutritional deficiencies.

Water (see also section on Water Quality) Aquatic animals need access to appropriately conditioned water. Fully aquatic animals obtain water in their habitat or absorb it across their gills or skin. Some semiaquatic amphibians and reptiles may need “bowls” of water for soaking and drinking, and water quality should be appropriate (see Terrestrial Animals section). Chlorine or chloramines may be present in tap water at levels that could be toxic to some species.

Substrate Substrates can provide enrichment for aquatic animals by promoting species-appropriate behavior such as burrowing, foraging, or enhanced spawning (Fisher 2000; Matthews et al. 2002; Overstreet et al. 2000). They may be an integral and essential component of the life support system by providing increased surface area for denitrifying bacteria (e.g., systems with undergravel filtration), and need routine siphoning (i.e., hydrocleaning) to remove organic debris. System design and species needs should be evaluated to determine the amount, type, and presentation of substrate.

Sanitation Sanitation of the aquatic environment in recirculating systems is provided through an appropriately designed and maintained life support system, regular removal of solid waste materials from the enclosure bottom, and periodic water changes. The basic concept of sanitation (i.e., to provide conditions conducive to animal health and welfare) is the same for terrestrial and aquatic systems. However, sanitation measures in aquatic systems differ from those for terrestrial systems because much of the nitrogenous waste (feces and urine) and respiratory output (carbon dioxide) is dissolved in the water.

A properly functioning life support system, designed to process the bioload, will maintain nitrogenous wastes within an acceptable range. Solids may be removed in a variety of ways, depending on the design of the system; generally they are removed by siphoning (hydrocleaning) and/or filtration. Depending on the type, filters need routine cleaning or replacement or, if self-cleaning, proper maintenance; in saltwater systems dissolved proteins may be removed by protein skimmers. Reducing organic solids limits the quantities of nitrogen and phosphorus that need to be removed from the system, both of which can accumulate to levels that are toxic to fish and amphibians. The biologic filter (denitrifying bacteria) typically removes ammonia and nitrite, potential toxins, from aquatic systems. Nitrate, the end product of this process, is less toxic to aquatic animals but at high levels can be problematic; it is generally removed through water changes, although large systems may have a specialized denitrification unit to reduce levels.

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