Question
I have 3 Koi in a 2400 gallon pond. I purchased 2 new ones from a local pond store. After aclamating them to the water, one of the new Koi, I never saw again. A week later the second Koi disapeared then a few days later one of my 18 inch Koi died. We thought maybe the big Koi ate these 2  small 3" Koi. A few days later my second large Koi died. The ph was pretty high but it always is on the higher side (pushing 7.9 - 8.0). I then over flowed the pond to change out some of the water with fresh well water. Then today our last Koi died - note that none of our 25 goldfish have died. What are your thoughts? Do yo think the 2 new Koi I purchased were diseased. What are your recomendations on purchasing more Koi? Any suggestions would greatly be appreciated.

Answer
I am so sorry about your koi.  It sounds like the new koi brought something into the pond that eventually killed the new koi as well.  It's not likely that the 18" koi would eat 3" koi.  They probably died and either the bodies have already decomposed, you haven't found them yet, the fish ate them after they died, or predators got them.

A pH of 8 is a little high but not a real concern.  My pond was up to a pH of 8.4 to 8.6 last week so I got it down to 7.6 with pH down.  The pond pH should be between 7 to 8 so a pH of 8 is fine.  It also wouldn't be deadly unless over 9 or below 6.

Since the goldfish have been unaffected, either they are more resistant to the problem, or it's one they don't get.   One such means of koi death is KHV or koi herpes virus.  It is basically 100% deadly to koi with no effect on goldfish.  It is treatable for koi by raising the temperature to 87 degrees F!  Go to http://www.koivet.com and enter KHV into the search box.  You will find many articles about KHV.  

I'm certainly not saying that your new koi brought in KHV but it's one possibility that bears mentioning.

I would wait for a few months to be sure that the goldfish do not become ill before purchasing more koi.  Keep in mind that if the problem was something contagious that happens to not be deadly to goldfish, they may still be able to carry that pathogen and then transfer that to newly added koi.  Be sure to quarantine your new koi for a month in a smaller pond (or aquarium if the koi are small).  If you do that, you might want to heat it to 87 degrees F to kill any KHV. Also, treat with pond salt at 0.1%.  In the pond itself with plants, you want to keep the salt level below 0.05%.  

You may also want to contact your local koi club for assistance.  They can sometimes test fish for you.  They may at least be able to do scrapings on the live goldfish to see if they're carrying any obvious parasites.

Good luck!

Robyn
fishpondinfo.com