Koi Carp all dying suddenly??

Question
QUESTION: Hi There
My dad has an 11,000 lire Koi carp fibre glass pond in the garden (UK) There is a large airstone at the bottom, a waterfall and skimmer and a sophisticated filter system About 3 weeks ago the biggest fish (about 2 ft) suddenly died. My dad tested the water and he said it was slightly out in terms of PH.
He also found some stones on the bottom of the pond, which had suddenly appeared from somewhere. He has got a hoover and regularly sucks all the rubbish of the bottom. It turned out it was the next door neighbours kids throwing them off the garage roof into the pond (darling children!!)
So far in the last 2-3 weeks 3 of the koi have died. On saturday my brother called me and said 3 more were either laying on their side at the bottom struggling to breathe or on the surface up side down gasping for air. After the last 3 died we turned off the waterfall as the fish were struggling and getting sucked into the skimmer.
I took a water sample to the fish shop and they said the water was too acidic. He also did a skin scrape from one of the sick fish but said it had no external parasites etc.
We did a 20% water dump Saturday evening and added one of the boxes of stuff to change the PH (all in the dark by torch light in the rain!!!!) We then added the second box of stuff first thing Sunday morning (we split it so it didnt change the PH too quickly) I then took another water sample to the shop and they said it was fine. By now at least 5 of the fish are laying on the bottom. Its so sad and i dont know what else to do. Can the change in PH affect the fish so drastically? Is there anything else i can do? I tried putting them in a salt solution for a couple of minutes ( a koi shop recommended this- i was desperate) As of last night only 2 Koi were swimming around, all beit lethargically!!!
The fish arent emaciated, havent got any visible signs of fungus etc, no bulging eyes, no bulging stomachs. Apart from them laying on their sides and not swimming they look perfectly healthy. Nothing has changed in the pond and no new fish have been added. The grass and garden hasn't been treated for anything and run off into the pond. Please help

Regards
Vicky

ANSWER: Hi, I have placed your questions on a koi forum where you will get the very best of advice because some thing is definitely wrong somewhere and the more answers that you get the better it will be that is why I have posted it on the Yorkshire koi forum so just log on to
http://www.yorkshirekoi.co.uk/index.php
and read the answers as they become posted by the members on "Koi keeper in trouble"
and I am sure between them they can find out what the trouble is. albeit a little late seeing you have lost so many koi.
George.

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

QUESTION: Thankyou. I logged onto lots of sites looking for some advice as it was so drastic. They have all died now except 1 and i think he is on his way out

Vicky

Answer
Hi,
So sorry for your fathers loss of his koi,
It certainly looks like the water has somehow been poisened for them to die as they have done, what colour sre the gill filaments inside the gills they should be a reddy colour if everything was normal.
George.