Koi carps usually don't lose their sight because they get a seizure because it's a common eye disease that doesn't cause vision loss. However, if the crystallites became serious, it might affect the visual sensitivity of the koi fish. If the koi carp also had signs of crystallisation in the other eye, it might cause the other eye to become less sensitive. Koi carp's crystallites would not be transmitted to other fish. Cataracts are changes in the crystalline lens inside the eye, usually caused by age, water pollution, excessive exposure to sunlight, or other factors. If the koi carps have serious symptoms of diabetes, it is recommended to send them to the veterinarian for diagnosis and treatment. The veterinarian may recommend medication or surgery to treat the cataracts.