Hi- just wondering- at one point, you had over 40 fish in the pond; what size is it (in gallons)?

2,800 gallons. It's pretty good sized and 3 feet deep.

90 degrees is hot but should not kill the fish. They would gasp for air if the oxygen was low and the big fish would go first. Did you test your water parameters?

Well, we're moving the fish out of that pond (which is mucky) and into a temporary pool while we clean out and fix the pond.

Some of the fish act just fine, and some of the fish are terribly lethargic. Once again though - no outward signs of disease at all - never seen something like this before. All the water got changed out this week when our pipe ruptured so there's good water in there that was treated. Our water has always been in perfect ranges. I think we're going to have to euthanize the fish that aren't hardly moving. I don't understand this - it's so frustrating.

2,800 gals is a nice size pond, but depending on the size of the fish, they might be overcrowded. The primary step toward determining whether the environment is too crowded is to do as another poster suggested and test all of your water parameters. I'd be especially interested in knowing what the ammonia number is.

You didn't mention what your filtration is like- if you can describe that also, that might be another potential source of finding out what the problem is.

Good luck to you though- it sounds like you have a lovely pond.

If I had to guess I would say you have at least one parasite. The warm water might have killed some of the fish because of a lack of O2 but once the temp dropped the fish would have been ok so it was not that. If it was poor water the fish would have responded nicely with the new water and they haven't. Since you lost fish last summer and fall it wasn't the winter killing them. But a parasite could and would have done everything you describe. I would treat with Proform C, Prazi, and Demilin and see how they do.
Mike

Mike- thanks so much for kicking this thread into gear.

What kind of parasite could have those invisible sytmpoms you think? And are you suggesting treatment with:

* ProForm C
* Prazi
* Demilin

...all 3 at the same time?

I thought I posted on this thread but I must be mistaken. I would bet the parasites came in with the new Butterfly Koi. I think you should read the posts in the following link to give you an idea about what to do and how to do it. There are some other posts that came before the link. Do a search on the forum for parasites and they should come up since I think they were the last posted. Sandy

Here is a link that might be useful: A previous discussion on parasite treatments

you can use prazi, proformC and dimilan at the same time. You cannot use ProformC if the water is below 60° though....it becomes toxic at those temps. Do you know the volumn of your pond? I mean exactly....not a guess? Parasite treatments can be deadly if you overestimate the volume and won't be effective if you underestimate.

Do you have rocks in the bottom of your pond, and if so, have they been cleaned lately? Rocks trap fish poop and uneaten food, and trap it while it rots. As debris rots, it can form hydrogen gas which is very toxic to pond life. It has a rotten egg smell.

I hope you find the cause of your fish death...it is sad when this happens.

Ccoombs1 is correct about ProformC to some extent in that below 60 it will go to the bottom of the pond and the level becomes too high and poison the fish but if the bottom and top water is being mixed all the time it is not a problem. Volume for the ProformC needs to be somewhat close with in about 10%. The prazi can be off even more and demilin can off even further. You have far more leeway with these than other treatments.
I don't think that it is HS or CO2 from decaying matter as not all the fish died at one time which is what I would expect with either of those. It would be possible that the levels could maintain right at the level to start to affect fish for a period of time but if that was the case than when winter hit and the pond froze over the levels should have increased to the point to kill all the fish. But it could be one of those poisons but not very likely. If I had to guess I would believe costia only because of the effects in cold water but it could another parasite that has weaken the fish to the point that the winter is finishing them off.
Mike

Actually, the problem with ProformC is not that it concentrates at cooler temperatures. The problem is the formalin in it converts to paraformaldehyde at low temperatures. This conversion occurs at temperatures lower than 60° (I believe it is around 50°), but to be safe formalin based products should never be used below 60°.

The thing that's confusing about all this, is we anaylsyed each fish as we moved them with intricate detail. There is most defaintely no signs of Ick, Flukes, or Anchor Worms. Nothing on the fins. Their scales were fine. The only issue I could see was a few of the fish had steaks (or capillaries) that were evident in their tail fin. I can't tell if it's supposed to look that way or if that's a sigh of illness. But the fish we had to euthanize because he was moving very little had quite a bit of red in his tail. Is that the sign of a bacterial infection?

Yes it does change to paraformaldehyde but at around 45 degrees but at lower temps but above the 45 degrees it does settle at the bottom of the pond and that can be a problem. But that is not important as it can cause problem at lower temps. Jpinard you do know that most parasites you can not see with the naked eye and must use a microscope to see them. Ick does not usually affect koi too badly and you might not even see a white dot the fish but it could be there. Anchor worm you can see but unless it is really bad the anchor worm probably won't the fish. The infection from it is a different story. The really nasty parasites you need a microscope for. The red in the tail is a sign of stress and is very common on butterflies.
Mike

Wow, I had no idea some of the parasites would need a microscope. I went to this site: http://mysite.verizon.net/bonniehill/ponds/pages/symptoms.html

and was confused when it emtnioend the red streaks in the tail and it said it was Hemorrhagic Septicemia - and then suggested a fungus eliminator? That just seemed odd for a bacterial infection?

VERY doubtful that your koi has hemorrhagic septicemia. Butterflys get streaking in their tails really easily. Parasites can cause streaking, but so can water parameters. Ammonia is a very common cause, as is mow pH or a pH that fluctuates because it is not buffered well enough. pH crash (or sudden dip in pH) can also kill fish. Can you post your test results? I mean the actual numbers? I am especially interested in pH, kH (total alkalinity), and ammonia.