I will give you guys the quick version since this is rather an emergency.

I have a small pond which was very overstocked. After 2 years in peaceful harmony, it all came to a crashing end this month, when fish started dying, one after the other.

Ammonia, nitrites, and nitrates appeared to be fine. And yet, every morning, more and more fish would die.

I would do water changes, but they still died.

Now, what I THINK happened is that 1) the warmer weather and overstocked pond made the diurnal pH swing too great. I literally would see the pH be around 7.4 in the morning, and 8.6 or so in the evening. I think this swing was too much and perhaps killed them outright, or weakened their immune system to where they succumbed and died.

But I digress.......

I THINK I have things under control now by adding a TON MORE AERATION to the tank, which is supposed to help with pH swings because it equalizes the CO2 in the water.

However, I have ONE DYING KOI. He's small, about 6 inches in size.......his first symptoms I saw was when he isolated himself from the others and only stayed on the bottom of the tank.

Then, ONE WEEK AGO, I saw that he could no longer swim right. He was swimming like a barrel roll - spinning in 360 degree turns.

I immediately moved him into a hospital tank, salted to 0.3%, and applied a treatment of Maracyn 2 (thinking perhaps it was a bacterial thing).

It's now been ONE WEEK and he is the same (if not, worse). He usually just stays completely horizontal on his side, breathing. He will occasionally swim but only on the bottom, and always canted to one side. When he stops swimming, he looks dead because he's on his side, but he is still breathing. I haven't noticed him eating at all, unless little specks of food get ingested while he's keeled over on his side.

Does this fish have any chance to recover? The fact that he's lived like this for a week is playing with my mind. I thought he would have died many days ago. So is there hope? Should I do anything else? Or just keep him isolated in this other tank until he either improves or dies?

I even considered putting him back into the main tank (because there is more aeration, more room, etc, but I don't want him to get picked on by the few remaining fish, nor do I want to expose him to those fluctuating pH ranges if the aeration doesn't control it.

Please let me know what your thoughts are on this.