Q : I’ve noticed that one or two koi with dorsal fins looked like bugs had eaten, but recently, there have been five or six. Could you please advise me on possible causes and solutions?

A : I think it is about missing sections in the dorsal fin, which could be caused by bacteria such as Columnaris (Flavobacterium columnare). Furthermore, given its transmission to other individuals, it’s highly probable that it’s an infectious disease.
As a course of action, it’s crucial to conduct medicated baths using antibacterial agents such as oxolinic acid or Sodium nifurstylenate as soon as possible to prevent further spread before it escalates.

Q : How long can koi survive in a bag with added oxygen?

A : It’s difficult to give a definitive answer since it varies depending on factors such as the size and quantity of fish and water temperature. For example, ten fish of 100g each would have a higher oxygen demand than a single fish of 1kg. In general, smaller fish have higher metabolic rates and require more oxygen. Lower temperatures tend to decrease metabolism, so koi can usually live longer in cooler water.
And pure oxygen is filled when bagging them, so theoretically, there is more than enough oxygen for several days. In cases where fish do die during bagging, it’s often not due to a lack of oxygen (although overcrowding can lead to oxygen depletion) but to other causes.
These cases include the accumulation of carbon dioxide and ammonia produced through metabolism. Regardless of the oxygen levels, the buildup of these substances in the water can result in fish mortality.
With elevated levels of carbon dioxide, koi will become anaesthetised.

Q : Please provide the appropriate and permissible values for pH, ammonia, nitrite, nitrate, and dissolved oxygen levels.

A : Information about water quality in koi keeping can vary significantly across various sources. This is likely because koi can adapt to various water conditions, and water quality can also vary depending on the purpose (such as raising them for health or aesthetic purposes).
For health, pH levels between 6.0 and 8.5 are considered acceptable, with a range of 7.0 to 7.4 being ideal. The acceptable range for ammonium (NH4-H) is below five ppm, with below 0.1 ppm considered ideal. Nitrite levels can be up to 0.5 ppm, ideally below 0.1 ppm. For nitrate, the acceptable range extends up to 10 ppm, with below three ppm being ideal.
Regarding dissolved oxygen levels, anything above 50% (around 4.4 mg/L at 20°C) is deemed acceptable, with an ideal range falling between 80% and 100% (approximately 7.1 to 8.8 mg/L at 20°C).
As a reference, water temperatures between 0°C and 38°C are adequate, with 20°C-28°C considered optimal. In addition, the level of COD (chemical oxygen demand, an indicator of organic pollution in the water) is considered acceptable if it is less than 4 pp. Please note that the standards for acceptable and optimal ranges may differ, but this is a compilation of various sources and should be regarded it as a reference.

Q : Please tell me the appropriate water depth for keeping koi. I’ve heard a trend lately towards keeping koi in shallower water.

A : I often encounter descriptions stating a water depth of around two metres, but it’s uncertain whether there is any scientific basis for this. Some references suggest that the minimum water depth should be deeper than the length of the koi. Koi naturally feed on bottom-dwelling organisms with their heads downwards. Consequently, if the water depth is shallower than their body length, their tails may protrude above the water surface, making it challenging to maintain their posture.