Sorry for your loss. It could be a couple of things. Have you added any new koi lately? Do you have rocks or gravel in the bottom...if so, how long since the rocks were removed for cleaning? Did you smell any rotten egg smell? Its doubtful the dealer can tell much from the dead koi. Parasites can only be detected on live koi. KHV needs testing at a lab. chemicals on the mulch is a possiblilty....but looking at the dead koi won't detect that either.

Well I checked this morining there are still 4 koi left alive. I did a 50% water change last night. Maybe they will make it. No new koi have been added in awhile. I did notice a couple of oil slicks on top of the water, not very big but deffinently chemical of some sort. No rocks in the bottom of the pond. I will contact the supplier of the mulch today just to ask if the know anything. Its strange all of my large koi died and the four that are left are small ones. I would think the opposite. Lee

Deactivated charcoal will remove whatever toxins may be remaining in the water. Get a lot of it....like a 5 gallon bucket full and put it in a net bag in a high-flow area. Sometimes the biggest fish are affected first because their respiration requirements are so much greater than the little fish. They are always the first affectd in cases of low oxygen. what is your water temperature and do you have aeration?

I called the place I got the mulch from they said as far as they know there is no dye or chemicals. I am not sure of water temp I live in sacramento ca. water probablly about 60 degress. Oxygen might be the problem now that you mention it. I have a large waterfall but I dont run all the time. when the water fall is shut off I have 7 jets in the wall for water circulation. I thought that might be enough for the koi. water circulation in pond is good. 55gpm. Lee

55 gpm is only 3300 gph....and that is not even close to enough circulation for a 13,000 gallon pond. You should turn the whole pond over close to once per hour. In cool water, oxygen is rarely a problem unless you are very overstocked. What kind of filtration do you have? Also...sores like you describe are often secondary to parasite infestations. If you took a dead koi to be examined, they would find no parasites because parasites don't stay on dead fish. You would need to examine a live koi for that.

I am using bead and glass filter with uv from sac koi.The one sick koi with the sore on his back I took in last week and they checked under microscope and found no paristies.

It doesnt matter what kind of filter you are using....you need to turn the water over close to once an hour. If the filter will only handle a flow of 3300 gph, then you need more filters.

Well the consensos is there must be some sort of weed killer and or dye in the bark. The wind blowing the bark into the pond was enough to kill somany so fast. I am doing another 50% water change right now. Hopefully the other koi will survive. lee