Fish dying in koi pond

Question
QUESTION: We have large koi pond (appr. 20,000 gals.)w/ top-of-line filtration system, bog filters, waterfall, water lilies, pickerel, etc.  Had 2 large koi (12 - 15"), probably 15 or so 4-6" comets and probably 150-200 offspring (1-2") from last year.  About 3 wks ago we purchased appr. 8 more koi ranging from 5" - 10" and appr. 10-12 comets ranging from 5 - 8", along w/ 2 new water lilies and a lotus plant. A wk. after adding these to pond, our pond guy added "Pond Tabs" to all pond plants, ckd. water pH, etc., and added salt, etc.  About 4 - 5 days later, one of the new 6" comets was found floating.  2 days later, found 3 of the 1" babies floating; next day 5 babies, next day none, next day 7.  Called pond supply store where we purchased new fish/plants and they sent someone out the next day.  She tested water and only thing she found was very slightly elevated pH and took water samples back for further testing.  Next day, there were about 40 dead babies. Pond gal returned and treated for pH but said all other test results revealed nothing(nitrates, etc.) and is confounded by why they are dying.  We ckd. w/ our landscaping co., and they state they only use organic fertilizers around the pond.  I'm concerned someone may have inadvertently used a pesticide or herbicide, but don't know how to check for this.  Since this has been gradually escalating over the last 2 wks., and none of the larger fish have been affected, would a pesticide/herbicide take fish out in this manner? Do you have any advice?  I'm in Atlanta and don't know if there's any store, agency, etc. that could ck water quality and/or do a "fish autopsy."  About 10 yrs. ago, we had a major fish kill and lost ALL of our large koi (about 25) and NONE of the small ones.  At that time, there was a pond supply store that took a slime sample and found chillodonella (sp?), but the owner/expert has since died.  We need help fast and would very much appreciate any advice or referrals you can give us! Thanks.

ANSWER: Dear Jan,

Sorry to hear this.
As I cannot see the pond or fish I'm going to estimate the risk in this matter to the fact young fish were added and after this many water corrections were made.
As you buy new fish you best check your water quality.
After the check and the water is ok you place the bag on the surface for some time so the temperature can be set to the same temperature as the pond.
Then you add some pond water to the bag and wait again..;
This makes sure there are no large changes in waterquality for the fish as they cannot stand to large water quality differences.
I think this is where it possibly did go wrong.
The fish were added and then there were water quality adjustments (pond tabs, salt, etc).
Maybe the change in quality was to large to take is and small fish tend to be affected quicker.
Certainly when they are just added to the pond and already stressed out.
Other cause can be less oxygen, however I think this is not the cause due to the fact that the larger koi are not affected.
Also, are the fish jumping, flashing, just hanging or visually injured? This can be A parasite or disease and control with microscope is needed.
I don't know for sure, however there should be A fish veterinarian in your area?
Maybe you can contact the local koi club for assistance also joe.hatfield2042@att.net  from atlantakoiclub.

Good luck!!!
Kind regards,
Raf

---------- FOLLOW-UP ----------

QUESTION: We followed all those rules when we introduced the new fish.  Perhaps you misunderstood, but we lost only one of the newly introduced fish (about 5-6" long)about a week or so after we purchased them. It was the first to die (that we noticed), but ALL the rest of the dying fish are the 1 - 2" long young that were hatched in our pond a year ago, so they were neither stressed nor relocated. Also, as I stated, there were no visible symptoms of injury or disease nor any unusual behavior.  We DID start increasing the amount of food pellets as well as introduce some larger pellets (for the larger koi).
I will follow up with the local koi club.  Thanks so much for your help.

Answer
Hi Jan,

I indeed misunderstood.
Sorry for that.
Then I think a good way to go is to contact the club to maybe come around with a microscope.
Then you can check for diseases and parasites first.
And then go forward from that point.
Good luck!

Kind regards,
Raf