Koi are quite sensitive creatures, and need the right pond temperature, pH level, and oxygen level to thrive.
Koi pond temperature
The best koi pond temperature is 8°-28°C , and they're happiest at about 15°-25° C. They can live through colder temperatures, but don't like to be at that temperature for too long – their metabolism slows and they stop eating, and their immune system shuts down, making them vulnerable to koi diseases.
If you live in an area with a long winter, it's best to make your pond quite deep – at least 1.5m – to protect them from the cold. When outside temperatures are freezing, the water at the bottom of the pond will be warmer, and the koi will seek refuge there. They can survive to about 3° C – colder than that will kill them. If you're concerned about extremes of cold, it is possible to get koi pond heaters.
When the koi pond temperature fluctuates a lot or very quickly, your koi will get very stressed and can develop a condition called hollow-back disease. The greater the volume of your pond, the less the water temperature will fluctuate, which makes for much happier koi. Also, the smaller the surface area of your pond, the less the temperature will fluctuate.
Oxygen levels for your koi pond
Koi need plenty of oxygen. One way to provide this is to get an aerator pump. A more attractive way to improve the aeration of your koi pond is a waterfall – but as you may want to turn this off for periods of time, is best not to rely on it solely, but rather use it to add to the work of your aeration pump.
Koi pond pH levels
pH is a measure of acidity and alkalinity. Koi are quite sensitive to pH, so you need the water in a koi pond to be about 7.5 – just a tiny bit more alkaline than true neutral (pH7).
It's a good idea to get pH testing kits from your local (or online) koi pond supplier, pet store or garden centre. Try to take your water sample from the middle level of the pond – not the surface, not the deepest part of the pond.
A really good koi pond filter plays an important part in keeping the pH level steady.