Question
We have a 10,000 gal./9 ft deep pond with a rubber liner that we just established last spring/summer 06. We had six koi fish, the largest being about a foot long in there that had 14 babies last summer. I took out the babies, as they were better off growing in an aquarium until they got bigger. Well, this winter all 6 koi fish died- we just found them a few days ago as all the ice over the pond now melted. The ice only got about 6 inches thick over the top this winter. We have an aerator that allowed a hole in the ice all winter. However, we did have quite a few leaves fall into the pond that we just couldn't get out before winter. I am so frustrated as this is my dream pond size, on the basis that I can leave them in there all winter-or so I thought. Our winter was not a very cold one- even for Wisconsin. Do you have any ideas as to why this may have happened.  

Answer
I'm so sorry your koi died.  Your pond is nice and big and deep.  You kept a hole in the ice which was good.  Unless you had a ton of leaves in there, that alone shouldn't have been the problem.  How far down did your aerator aerate?  If it was only on the surface, I suspect that down in the bottom where the koi were was not high enough in oxygen.  On the bottom, the levels of carbon dioxide from the fish and hydrogen sulfide and methane from the rotting leaves may have also been high.  If you didn't, I suggest in the future dropping a large air stone thing to the bottom of the pond to aerate the bottom.  Sure, this may bother the fish a little as far as movement and noise but it will keep them from suffocating.  I have a 153 gallon pond that I kept a de-icer in but the frogs kept dying overwinter until I dropped an air stone to the bottom in the pond.  Companies sell aeration systems for ponds.  Here is an example of an aeration kit for ponds:
http://www.drsfostersmith.com/Product/Prod_Display.cfm?pcatid=8556&N=62728+11336...

I hope that is why your fish didn't make it because that is easy enough to fix in the future.  Did the fish show any signs of abnormalities?  Larger koi need more oxygen than the little ones.  Again, I'm sorry for your loss and hope things improve for you!

Robyn
fishpondinfo.com