There are three basic types of aquatic systems, broadly defined as “open,” “semi-open,” or "closed." An open system has incoming water from some source (eg, surface water, well water, or city water) flowing through the culture facility one time, then discharged. Raceway and cage systems are examples of open systems. Semi-open systems have some capacity to recirculate water, which requires some type of treatment, but fresh water from an external source may be added as needed to supplement the treated water. Closed systems can be natural, such as static outdoor ponds, or can be highly engineered; intensive aquaculture systems depend on extensive filtration of water before recirculation. Aquariums are typically considered closed systems; however, the complexity of design and carrying capacity of individual systems vary greatly.
Basic housing units for fish may be outdoor ponds, raceways, cages, or aquarium systems (which may include recirculating system designs). The selection of a housing unit will be determined by available resources and facilities, as well as the goals of the owner.
Outdoor ponds are typically constructed in clay-based soils, or they may have a type of plastic liner to retain water. In Florida, there are also “water table ponds,” which have a certain amount of horizontal flow of ground water into and out of the pond. Production ponds vary greatly in size, but a good design maximizes surface area while minimizing depth (ideally <6 ft). This design decreases the risk of stratification that can result in catastrophic “turnover” events (see Dissolved Gases). Ideally, an aeration device should be available for all outdoor ponds. In the southeastern USA, most aquaculture production ponds range from 0.1–20 surface acres of water, whereas recreational fishing ponds usually have a smaller surface area but may be quite deep, substantially increasing the risk of stratification and turnover.
Ornamental ponds may vary from a few hundred gallons to many thousands. Larger ponds (>10,000 gal.) are usually easier to manage from a water quality perspective and are more forgiving of an owner who may overstock or overfeed the fish. Most larger ponds have water provided from a well or city water source and are operated as closed systems. Supplemental aeration is also very important for these systems and is often provided by a waterfall, stream, or other aesthetic means of moving water. Clinicians should not only test dissolved oxygen (DO) when evaluating an ornamental pond but also assess water movement. In poorly maintained systems, filters will become clogged and flow rates can decrease dramatically. Poor flow rates result in poor performance by biofilters and, ultimately, system failure with consequent rises in total ammonia nitrogen concentrations. Poor flow rates are also often associated with low levels of DO and an inability to maintain oxygen concentrations near saturation. If the pond depends on a waterfall as the primary source of aeration, and flow rates decline, the aeration capability is also compromised. Shade may be an important consideration when evaluating an ornamental pond. Lack of shade can result in rapid and extreme heating of water, especially if the pond is shallow. Hot water is not only detrimental in its own right, but it also does not hold oxygen well, increasing the risk of oxygen depletion. Shade trees around a koi pond may contribute to leaf litter and organics in the pond. Preventing predation is another important consideration when designing an ornamental pond. Typically, fish housed in these units are very colorful and may be very attractive to birds (including owls), as well as to mammalian predators such as raccoons or otters. In Florida, reptiles (eg, alligators) may also prey on ornamental fish. Some type of visual barrier to minimize the detection of colorful fish by birds can help, as can pond design features or other structures that limit the ability of wading birds to access the pond. An electric perimeter fence located 12–18 in. off the ground may keep small mammals or crocodilians out of an owner’s yard, protecting the fish and other pets.
Other types of systems, more typical of aquaculture production, include cages and raceways. A raceway system is typically a series of long, narrow, and relatively shallow concrete or earthen tanks. Water enters the unit at one end and is discharged at another. Often, these use some source of surface water, such as a flowing river, for growout of the fish. Advantages of raceway systems include capacity for heavier stocking densities than ponds of a corresponding volume. Disadvantages include concerns about bringing pathogens or contaminants onto the farm and discharging nutrient waste, pathogens, or treatment chemicals in the effluent. Cage production is most common in open water, with the salmon industry being a prime example. Typically, large cages are placed in protected bays for the grow-out phase of the operation. Alternatively, new technologies allow cage systems to be completely sunk in deep water, decreasing the risk of damage from wind and wave action. Advantages of cage production include tidal water exchange, which would not require pretreatment to be suitable for the fish. Disadvantages include potential damage to equipment or escape of fish during storms, potential contamination in the event of a chemical spill or harmful algal bloom, and concerns about adverse environmental impacts by the aquaculture operation itself. Global use of cage culture technology is increasingly important as marine food fish are cultured more intensively.