How big is the pond? How long have you had the big koi? Are there plants in the pond?

Did you quarantine the new fish before adding them to the pond? Perhaps the newbies brought in a disease. If the water quality wasn't fantastic to start with, the big koi could have been susceptible.

I know virtually nothing about koi, but I have some experience with aquarium fish. I expect that the guidelines are the same, only on a bigger scale - so many gallons per fish, water changes, don't over-feed.

Sorry you lost your fishies - I hope you can figure out the problem.

Make sure the pump is running and not clogged. They need oxygen just like we do.

This post was edited by EmmaR on Thu, Oct 9, 14 at 17:29

Water temperature?

Size of pond?

Size of fish?

Filtration?

Water changes?

Temperatures of water and air?

If you want more help, we need more information.

Are they coming to the top of the water and gasping for air. When my pump failed that is what happened before they died.