On the little you give us, I would guess parasites. Can you give us any other information at all?

Sandy,I think you are right, either parasites or a bacteria problem but I would suggest that the poster should go directly to Dr Johnson at Koi Vet (the site is up and running again) for assistance.
Unfortunately, it sounds like the problem is well advanced with that many fish dying each day.
"Horton"

Here is a link that might be useful: Koi vet site.

When I first started ponding I had fish dying everyday that had chunks out of their tails or fins. Used a net and scooped out a couple of those big beetle things with the pinchers on the front--not a water boatman another water beetle that gives a nasty bite. They like to hide in the bottom of the pond. No fish loss after that.
Do you have enough aeration in your pond? If you have a lot of fish and the weather has been hot the oxygen may be getting depleted at night time.

There are a number of things that can go wrong if we are not vigilant at the pond. The best defense is being observant and informed. Koi Vet is one of the best resources when our fishies are in trouble. The parasites are sneaky and it can be hard to see them even when you know what you are looking for. Bacterial diseases can lurk in the old mulm that collects in and under rocks if you have them on the bottom. Both parasites and bacterial diseases can show up as the water gets warmer. Mud can harbor gram negative bacteria which is virulent to fish. Dead leaves on the bottom harbor both. Fish jumping out of the water can indicate they are being attacked. Dr. Johnson's Koi Vet can help you diagnose the problem and prescribe a solution. Thank you Horton, I wasn't aware they were back after a short leave of absence. I'll go there later to see what the new site looks like. I'm sure they have made improvements.

Sandy, Dr J certainly has done a great job with the new site.
There's lots of information to be had and instructional videos to look at.
I found that this new site is much easier to move around than the old one.
"Horton"

I know this is a year old topic but I just wanted to share something that happened to us last night. We have our pond with 8 fish in it. (4 Koi and 4 commets) We also have a rain barrel that we have a few feeder fish in. There is no aeration. Yesterday it got very hot here and the rain barrel water got really warm. I didn't expect the fish in there to survive and started wondering how they'll survive a hot summer. The previous owner put feeder fish in there every year to help with the mosquito larvae so we did the same. The fish were all near the bottom of the barrel, I guess because it's cooler. Later in the evening, my daughter came crying that one of the fish had died so we went to scoop him out and noticed that there was a large beetle (almost the size of the fish) ATTACHED to the fish. It was eating the fish!! I feel bad for the poor little thing and wonder what's going to happen to the others but more so worried about my pond! Can this happen to my pond too??

Lori, I could be wrong, but I think the beetle probably attached itself to the fish after it had died. The fish was probably floating, and made it easy for the beetle to get to it.

That makes sense Nancy... I sure hope you're right. It was so sad to see though. Even though it's only a feeded fish, I felt bad for the poor thing.

Can't figure it out. I have a small pond ,several years old now. Plants are arn't doing so well, dying off, leaf lettuce has shredded edges, the water lilly is rotting and the water hyachins are browning around the edges..No mud or rocks for bateria or paracites to harbor..PH is fine...Sooo why the dying fish? Water has been changed and settled for over a week, Any suggestions? Just regular gold fish, usually hardy in past years.