Well, all the koi are dead now: the pretty 12" ones I had for six years and
the 3 new ones added this summer. They seemed to die one at a time, from
largest to smallest animal.

None of the casualties showed any signs of bruises, parasites, fungus, etc.
They just stopped eating, became listless, eventually had problems with
buoyancy, and died.

The two goldfish which have been in the pond for a long time still seem
fine.

To help analyze this and perhaps to serve as a warning, I will list the
steps which led to this end and recount all of the changes to the yard/pond
made this year. If anyone can point out something, I would appreciate it.
I have a beautiful empty pond which I would love to restock with small koi,
but I am a little afraid right now.

Changes in the Fish Community
1) I had 4 12' koi (raised in my ponds for 6 years) and a couple of 4"
goldfish in the big pond together for more than a year now. I had the big
pond built last summer as a home for the big guys.
2) About a month ago, I picked up 5 small koi (part of the deal I got with
my pond). I put them in my old pond with some goldfish for about 10 days.
These fish were not great fish, but they were free. Two died pretty quickly
and the rest were very jumpy. But they looked OK so I put them in the new
pond.
3) The new koi all died in the new pond over the course of about 10 more
days.
4) Everyone left seemed OK so I added 3 more 5" koi (from a better dealer -
these ones were not so jumpy and looked very healthy).
5) About a week later, I noticed that the big koi were not eating normally.
Which is to say that they were not constantly ready to eat. Generally, if
you kept ladling food, they kept eating it.
6) The big koi died first then one died every other day - pretty much
largest fish to smallest fish (but only the koi). No visible parasites,
wounds, fungus, etc. although their eyes looked funny. They became listless
and hovered rather than swam. They spent a lot of time at the bottom of the
pond in the the heron shelter I built. At the end, the floated on the top,
not moving at all (as if paralyzed), just breathing.

Changes in the Yard
1) Added more mulch around the pond (from a different nursery than last
year).
2) While digging in the yard, fed any earthworms I found to the koi (I don't
use herbicides or pesticides).
3) Added a mosquito magnet about a month ago. This burns propane to create
CO2. This was about 15 feet from the pond at a lower level than the lip of
the pond. (Note: this is about 20 feet from the old pond which is full of
goldfish and has not had these problems).
4) I noticed that the pond has a frog visitor.

My Ideas
1) I introduced some strange koi disease with the new fish. It would be
something that does not leave a mark and strikes the largest animals first.
2) The worms were toxic, leading to a slow death for the koi.
3) The CO2 plume emitted by the mosquito magnet created a CO2 pool over the
pond, resulting in a low oxygen situation. The fish were gradually poisoned
by the gas. The larger the respiration system, the faster the succumb.

Any Ideas would be appreciated, especially advice on how to safely start
over.

Thanks,

Rich in Chicago