Rapid Losses In Koi & Goldfish

Obviously "time is of the essence", but that does not obviate the need for immediate water quality testing. When fish are sick and dying rapdily, my FIRST thought is water quality, to include poisoning or derangements with Ammonia, Nitrite, Nitrate, PH and Carbonate Alkalinity. Even issues with carbon dioxide and hydrogen sulfide are possible. However those two parameters are rarer, and much harder to test for.

So, the first steps with rapid losses include water testing.

If water quality appears "good", some would recommend a massive water change to remove any possible toxins, reduce bacterial numbers, thin populations of parasites, improve overall water quality including ORP, and more. I would endorse the concept and perform a major water change if it would not seriously alter the ambient temperature of the pond in the 'colder" direction. (Say, mid summer you have to be sort of careful).

After the water change, the fish might appear better but you'd be smart to dispatch one or two fish to KoiLab for testing. Some of this can be done free of charge.

If you're concerned that it could be a virus, for example if you've recently added new fish without quarantine and now fish are dying pell mell - consider this article on the Koi Viruses, and then hasten to koivet.com for more on the viruses.

Then, salt at 0.3 to 0.6% would be highly recommended. Read on.

If you want to try a shotgun (blanket) regimen that could work well, after the initial water change:

You'd be VERY smart to consider a shotgun remedy for basic diseases, plus superheating, to skate past various parasite, bacterial and KHV (virus) diseases.

Recommendation then:

1. Feed medicated food for the first 14 days of quarantine
2. Use salt at 0.3% to 0.6% (safe for all fish) (Salt article)
3. Use Prazi in case (good chance) there's Flukes on the incoming fish.
4. Heat for ONE WEEK to 86 degrees Fahrenheit. (Article on heating.)