Diseases or Disorders of the Eyes of Koi
Koi Care - Cloudy corneas (eyes look whitish or white)
When you see your koi with cloudy or white corneas you have to wonder if your koi has hurt itself, is being hurt by the environment or is being hurt by something you're doing.
We see cloudy corneas after misapplication of SupaVerm, Formalin, Potassium and any other compound that is caustic in concentrated form. Cloudy corneas can also be caused by bacterial infections. In these cases, antibiotics injections, or medicated koi food can put the brakes on this.
Koi Care - Eyes are bulging
Bulging Eyes are not a very good sign. In Koi which are scaleless, this is sometimes the only way to tell your koi is terminally dropsied. In other cases, the eyes may bulge during a salt treatment combined with an organophosphate. We don't usually recommend the use of the OP's because of toxicity issues but sometimes they have to be deployed.
Popeye, or exophthalmia is the condition of an eye wherein it protrudes from its normal position in a socket.
For years in the literature, the causes of exophthalmia were few, and almost all literature surveyed suggested it was a mycobacterial infection, or Tuberculosis forming an abscess or granuloma behind the eye.
In this case, that may very well be true, and you should be aware that there are few drugs effective against piscine tuberculosis, there are even fewer that can access the space behind the eye. In humans, these infections are still hard to treat becasue the space behind the eye is isolated from real good vascular supply. As well, tuberculosis, particularly the one for fish, is transmissable to humans through open sores. I'm not saying it's always TB, but it could be, so avoid contact.
Other causes of pop eye include viral and other causes of inflammation to the Choroid or vessels leading to the eyeball itself, and trauma ranks high in this group. If the eye does not get picked OFF when it protrudes like this, it is entirely possible that it could actually pull back in as the swelling behind the eye reduces.
Let me say a word or two more about bacterial or TB pop eye. It spreads slowly, and gets its foothold through stress. Be sure your pond water quality is optimal, and that the diet and temperature are perfect for the chosen species you maintain. You can try antibiotics with this, even injected ones if the fish warrants it, but I really suggest you not get your hopes up.
Koi Care - Eyes are sunken in
Wow. Sunken eyes are usually associated with Aeromonas / Pseudomonas bacterial infections which have gone systemic (gained entry to the blood stream) and sometimes you may see fish which have been parasitized by Flukes, for example, to such an extent that they have essentially bled to death through the gills and skin. The loss of protein and plasma in the fish leaves them wasted and emaciated and the eyes will sink in. Once it gets this far, the prognosis is extremely poor. I've never seen a fish with sunken eyes to survive. Never. If the fishes' head is shaped like an arrow head, and it's shimmying in the water, and the fins are clamped, the fish is a goner. This is disturbing, of course, but don't let it stop you from saving the fish that have not gotten this far! You owe it to yourself, and to the remaining fish to aggressively treat. Here's what you do: Salt the main system according to these instructions. Inject any fish which are valuable. Try as much as possible to make a diagnosis of a parasitism if possible using a microscope. If this is not a possibility, a shotgun remedy could be considered.