Question
I have a pond about 10,000 gallons, outdoor, unfiltered,but well-circulated, moderately green, but chemistries (nitrates, ammonia, pH, salt) all ok and stable with many comets, about 30 koi, one snapping turtle and many frogs. For the last 5 or 6 weeks I have been finding 1 or 2 dead adult goldfish every few days with no visible signs of disease or injury, and the koi and the undead comets seem to be fine. I have just finished the third weekly treatment with paracide green, but found another dead comet last night. Will appreciate any ideas you have about what kills goldfish but not koi or suggestions.

Answer
I'm sorry about your goldfish.  That's a nice big pond!  Is there a reason you don't have a filter (too far from an outlet)?  How big are the koi?  30 seems like a lot even for your size pond if they're full size.  A few years ago, I lost 13 goldfish over 3 months.  My koi and orfe and other animals were not affected.  I never knew why for sure.  Water quality issues should normally affect all fish.  Recently someone suggested it was copper in my pipes but I doubt that.  I don't think it's parasites but it could be.  Since parasitic medications are so strong and can kill some animals, I suggest avoiding them unless you know for sure they have parasites.  If you contact your local koi club, they can help you do scrapings of the fish and look under a microscope for parasites.  

You say the pond is circulated.  Does that mean it does have electrical components and aeration?  If so, then oxygen should be ok but if it's been really hot there like here, it could be low oxygen.  Smaller fish have less demand for oxygen but often die first for some reason.  I suspected low oxygen when my fish died but it tested fine.  

I wish I had a fast and exact answer for you. The truth is that sometimes our fish die, and we never know why.  I would test the water for pH, hardness, alkalinity, ammonia, nitrite, salt, and oxygen (you've done some of those) to rule those out as problems.

Finally, sometimes predators beat up fish but don't kill them or leave obvious marks on them.  A raccoon can chase a goldfish literally to death.  Herons can beat them up too and may not always eat their victims, especially if they don't fit in their mouths.  I would think if the snapper went for the goldfish, there'd be some bodily injury.  

Those are the things that come to mind now.

Good luck!

Robyn
fishpondinfo.com