Whether you measure your pond's temperature in degrees Centigrade or degrees Fahrenheit or both, a thermometer is considered a requirement for all ponds. A floating pool or spa thermometer is good. It is recommended that it be floated in the filter/converter system or tied to an easy access point at the edge of the pond. At a slightly higher cost, the electronic indoor/outdoor thermometers on the market (i.e. Radio Shack) provide a continuous digital readout. Just drop the end of the waterproof outdoor probe into the water. (Note: Small floating glass aquarium thermometers have been swallowed by Koi.)
Temperature Ideal Range 65O F-75OF (20OC-25OC)
Acceptable 35OF-85OF (2OC-30OC)
The temperature of the pond normally follows that of its surroundings although with a delay related to the size of the pond. Direct exposure of the pond to open sky can cause larger swings in temperature. Direct sunlight during the day can cause the temperature to rise higher, and heat loss on clear nights can cause the temperature to drop lower than shaded ponds. A clear night sky can absorb a large amount of heat from a small pond and actually drive the pond temperature below air temperature.
Events generally happen faster at higher temperatures and in smaller ponds. Over normal temperature ranges, biologic activity doubles for each 10O rise in temperature. The toxicity of ammonia increases as the temperature rises and the amount of dissolved oxygen that the water can hold decreases. Although Koi have been known to survive for limited periods at 100OF and even higher, the mortality rate of fish conditioned to 75OF water increases rapidly above 85OF. Above 80OF, supplemental air may be required. Below 55OF (12OC), Koi stop producing antibodies and at about 45OF (7OC) enter a state similar to hibernation. Bio-converter bacteria activity ceases at about 40OF (5OC).
Feeding fish versus Temperature:
Less than 50OF Do Not Feed
50OF-60OF 2-4 times weekly
60OF-85OF 2-4 times daily
Above 85OF Do Not Feed
In all cases, try to feed only what the fish will normally consume in about 10 minutes. Remove any uneaten food within an hour.
Fish do not like changes in their environment of any kind, including temperature. Any changes add stress to the fish and the larger and faster the changes, the greater the stress. This is considered by many to be the primary reason that fish do better in larger ponds. Another time that the Koi are subjected to stress from temperature changes is when they are being transferred to a pond from another location. My recommendation is that if the fish have been bagged for more than four hours, it is better to release them immediately than to subject the fish to the "bad" water in the bag for an additional half-hour. Thirty minutes of floating will prevent a sudden shock if the temperature difference is large, but it will not acclimatize the fish to the new temperature. Actual temperature acclimation of a fish takes several days, similar to us dealing with jet lag. It is not only the temperature the fish need to be accustomed to but also the pH, hardness, alkalinity, "the taste", etc. of it's new surroundings.
Other than providing some shade (summer and winter), little can be done or normally needs to be done to control an outdoor pond's temperature. A waterfall in dry climates can provide significant "swamp cooler" action and a large waterfall can provide considerable cooling (at a sacrifice of additional water evaporation losses). This same action can occur in the winter time as well and should be taken into account during the winter in cooler climates.