A thorough understanding of the life-support equipment with clinical significance is necessary for the pet-fish practitioner. Along with the physical exam and diagnostic workup of the aquatic species and water quality assessment, the assessment of the design and function of the aquasystem, along with the equipment, is an integral part of the case workup. This usually requires an onsite visit, especially for koi medicine, which will account for over 75% of the caseload of the pet-fish practitioner. Along with examining both the mechanical and biological filtration, of critical importance is the understanding of the circulation, gas exchange, and temperature control and the interactions between these factors.
In reviewing ponds, a primary assessment involves the nutrient balance. Ponds may be divided into three groups: water gardens, characterized by small decorative fish species, an accent on natural appearance, having much of water surface covered by plants and no feeding of the fish; the goldfish pond, with small decorative fish species, the water surface largely open to view, and with regular feeding of the fish; and the koi pond, where koi and/or other large species are kept, the water surface being largely open to view, with regular heavy feeding of the fish.
The next assessment involves the basic design concept, of which two types dominate. The first is the shallow dished pond. Its attributes are a traditional natural appearance, easier to build with a cost about one half that of a deep pond. It utilizes "in pond" settlement of waste, its biology dominated by a very high number of microorganisms along the bottom. This type is best for a water garden or goldfish pond, as it has a high stocking density when koi are kept. Negative aspects are that it's prone to unstable water conditions (temperature, oxygen levels) and requires regular clean-outs when koi are kept (yearly cost). Heron predation can be a problem. The alternative is the deep pond. Here, the circulation and filtration systems are designed for efficient high water quality production with more stable conditions. This is the best type for koi ponds, as it has a lower (half) stocking density of a shallow pond of the same footprint. The equipment creates a system that is self-cleaning (no yearly maintenance costs). Heron predation is minimized. The negatives are the equipment and installation cost double that of the first and a more formalized appearance. Most large ponds of 50,000 gallons (200,000 l) or more are of the first type. They tend to have a low stocking density. Circulation and gas exchange remain important nevertheless. This is often accomplished via fountains or wetlands filters.
The process of gas exchange is critical and is assured by air diffusers and stones, waterfalls and spillways, surface skimmers, fountains, venturis, and pond deicers. Water temperature is moderated by ambient air temperature (poor control), via a greenhouse, which is better, but still variable and with heaters. Mechanical filtration occurs through sedimentation (no filter), via internal box filter (submersed near the pond bottom), external box filter (outside the pond), prefilter solids separator, either with a sedimentation chamber (vortex) or via a pressurized unit or a pressurized canister filter, of either downflow design (may contain sand, pebbles or plastic media) or upflow design, with floating bead filters. As to biological filtration, these can be independent units, but most are combined with mechanical filtration. It is important to always identify the biological portion of the filter. Concerning circulation are the mechanism, pattern and standard parameters to know. As to the mechanism, active circulation is created via pumps (either external or submersible), fountains and air lift via a bubbler. These last two methods interfere with the settlement of suspended solids and are less desirable when filter systems are used. Circulation can also be passive, where a pond deicer is used in winter. This method creates a vertical circulation, so should be placed directly over where the fish congregate and should not be combined with any other circulation mechanism. The parameters to know are the flow rate, which is described as the "turnover time." It is the system volume divided by the true output of the pump. This is the advertised output minus loss due to total dynamic head (discharge height + pipe friction). It is important to determine the true output (GPH) by examining the return(s). For warm season flow, the rate in shallow ponds should be ½–1 h, in deep ponds, 1–2 h. In winter, the turnover time should not be less than 10 h. As to the circulation frequency, it should be continuous.