Question
Hi Robyn,
I Have a 1000 Gallon Pond with skimmer, waterfall filter and canister filter
with UV. I also have various plants 2 water lillies, parrots feather, canna,
lettuce, haecinth, grasses, & lotus. I also have about 15 small gold fish & Koi
3-4". It is rubber lined and also lined in cobble stone and pebble. My problem
is I just had 3 of my large koi (8-12") die all of a sudden within 1 week , but
the small one's seem fine. The water tests fine for everything. I did look for
wounds or fungus, but could not see any sign. Could my water be
contaminated?

Answer
I'm so sorry about your koi.  Have you made any changes lately such as added new fish?  The larger koi are more sensitive to certain things.  For example, if it's been overly hot or overly rainy, the lower oxygen levels in a pond could have killed them even with a waterfall going.  What water tests did you do?  What were the results?  I suggest pH, hardness, alkalinity, oxygen, ammonia, nitrite, and nitrate.  Do you do water changes? Do you add anything to the pond such as dechlorinator or salt?  I suggest doing a 10% water change and adding dechlorinator and salt at 0.05% which won't harm the plants but may help the fish.

The good news, if you can call it that, is if the water were truly poisoned by say bleach, soap, pesticides, etc., then all the animals would have died not just the big ones.  

Also, even if a fish seems ok on the outside, you can't tell if it had microscopic parasites, bacteria, etc.  You can consult your local koi club to examine the koi, seek out a koi vet, or even mail frozen koi for autopsies.  See http://www.koivet.com for more on koi health.  

The koi experts say to have 1000 gallons for the first koi and 100 gallons for each additional.  Since you have more, who aren't really huge (the 2-3 foot long koi) but not tiny either, there's a concern that the biological filter can't handle the load, or there may not be enough oxygen.  I suggest adding an air stone to the pond whether or not you can test the water for oxygen.  They sell a test kit for oxygen for under $10 so it's good to rule that out (although the oxygen you have now may not be what was there when the koi died).

As far as water contamination goes, there could be levels of something bad that were enough to kill the larger koi with their larger oxygen demands but not enough to kill the smaller fish.  Those bad chemicals could include ammonia, nitrite, copper, other heavy metals, pesticides or other chemicals from runoff, etc.  To test for the other things would require hiring a lab which is not cheap (I work for one actually that mostly tests food but I can get my water checked for free).  I hope no more fish die.  My pond has lived through one die off a few years ago of 14 goldfish with no clues at all (including tests at work like pesticide screenings) (and no effect on the koi, orfe, frogs, etc.) so I know how it feels to feel helpless.  Good luck!