Many factors must be analyzed when doing a complete veterinary diagnostic workup for a pond problem. In areas where two distinct seasons are characterized by a warm metabolically active and cold metabolically inactive period, the diagnostic workup should account for these variable factors. Despite a relatively low utilization rate and high water concentration of oxygen during cold weather periods, difficulties maintaining ventilation of pond water during freezing periods can lead to hypoxic conditions. Furthermore, several mechanisms, such as pond deicers, heating systems, air bubblers, and high circulation rates with spillways, which have been promoted to mitigate winter hypoxia, can themselves lead to medical problems. A thorough understanding of the inter-relationship between air and water temperature, water circulation and gas exchange is critical in evaluating cold-season health problems. In addition, two distinct pond-design formulas are currently in usage in the USA, that being the deep pond with circulation featuring bottom drainage and the shallow dished pond with surface circulation only. As each of these differ metabolically, an understanding of these design features is important. Gas exchange for ventilation may be accomplished by using air diffusers and stones, waterfalls and spillways, surface skimmers, fountains, venturis, pond deicers, and slow bottom to surface water flow rates. As koi have adapted to overwintering at the bottom of lakes and ponds at a water temperature of 39°F (4°C), colder water can lead to medical problems such as koi winter "layover" syndrome. Water temperatures as low as 28°F (-2°C) have been recorded by this author. Water temperature can be modified, intentionally or not, by the use of a warm air bubbler, greenhouse, solar cover, heaters, and pond deicers. A simple waterfall bypass has been used successfully to reduce pond water temperature fluctuation during periods of varying air temperatures. Circulation is accomplished actively via external or submersible pumps, fountains, air lift devices and passively with pond deicers. The rate, called the turnover time, and the frequency should be analyzed. Problems encountered are often caused by too high a circulation rate or the use of multiple devices. Water chemistry is less often incriminated as the source of cold weather problems. However, due to the lack of intentional water exchanges during cold weather, depletion of carbonate buffers will lead to pH instability in those areas where water is naturally soft. Other conditions seen during cold weather are entrapment in shallow areas, saprolegniasis or water mold, a fungal pathogen that grows well in cold water and Ichthyobodo, which can reproduce rapidly in cooler water, more so than is often seen with most parasitic infestations.