Q : Biologically, koi are not known to undergo sex changes. However, when talking to koi breeders, I’ve heard cases of females turning into males and vice versa. What is the actual situation?

A : Indeed, koi are generally not believed to undergo sex changes. Therefore, it is possible that the breeders may be mistaken (I have also heard this multiple times).
However, since they are living organisms, we cannot completely rule out the possibility of irregular individuals. There have been records of individuals possessing both testes and ovaries. I have personally seen such an individual, though not in Nishikigoi but in Magoi. However, whether that individual was sexually mature is unconfirmed. No scientific evidence supports this, so it’s hard to say definitively. However, it seems we might be in an era of diversity for koi as well…

Q : Around mid-March, one of my koi suddenly started showing signs of poor health. I haven’t fed them at all from December to May. I suspect it might be gill disease, but can they contract gill disease even without being fed? Additionally, I disinfected them before stopping the feed.

A : Please note that it’s hard to say definitively due to the limited information provided. First, you mentioned the koi is not doing well, but could you refer to any specific symptoms? Additionally, details about the conditions in which the koi are being kept would help diagnose the issue and provide a more accurate response. Since you haven’t been feeding them this time, I’ll assume they are being kept outdoors.
While referred to as gill disease, there are multiple causes: bacterial, such as Flavobacterium columnare causing Columnaris disease; parasitic, including the ciliate Trichodina causing Trichodina disease; monogenean Dactylogyrus causing Dactylogyrus disease; and myxosporean Myxobolus causing Myxobolus disease.
Assuming an outdoor water temperature of around 10°C in mid-March, Columnaris disease, Dactylogyrus disease, and Myxobolus disease are less likely candidates because they typically occur at relatively higher water temperatures. Therefore, Trichodina is a likely possibility if it is a gill disease. Trichodina infection causes symptoms such as reduced feeding, slow swimming, and severe bleeding from the gill covers and fins. A heavy infestation of Trichodina in the gills can lead to increased lethality and is highly dangerous.
Medicated baths using pigments such as methylene blue or malachite green are effective as a countermeasure. If treating the entire pond is difficult, consider taking them out individually for medicated baths. Additionally, using a saltwater bath (0.5%) can also be effective.

Q : Are there any alternatives to oyster shells? How about organic lime?

A : First, concerning the effect of oyster shells, their primary function is to help stabilize water quality by preventing pH from dropping too low. The water in the aquarium accumulates toxic ammonia due to leftover food and fish waste. In an aquarium where beneficial bacteria (Nitrosomonas and Nitrobacter) have been established, Nitrosomonas bacteria convert ammonia into less toxic nitrites, and Nitrobacter bacteria further convert nitrites into nitrates. Therefore, in aquariums without water changes, nitrates accumulate over time.
Nitrates are less toxic than nitrites, but they are acidic substances, so they gradually lower the pH of the water. At this point, oyster shells release dissolved calcium into the water, which helps prevent pH from dropping. Another material that serves this role is coral sand. Like oyster shells, coral sand is composed of calcium, similarly stabilizing pH levels.
Organic lime is derived from shellfish shells, such as oysters and eggshells so that it may have similar effects. However, finely powdered forms of organic lime can dissolve calcium more readily into the water, potentially causing excessive pH levels, so caution is advised.