Pond and Fish deteriorating

[Mina]

I have a 3 foot deep outdoor pond with comets and koi about 6 years old. The comets have started to die, first one now and then, but now quite a few. I'm in Maryland and it's been a mild winter. I have a leak and have been feeding water in constantly from a well. It used to fill from our big spring fed pond. It's aerated and they have been fine previous winters.

 

[MitchM]

Have you done any water tests?
Can you describe the filter setup and water circulation?
What are the other dimensions of the pond?

 

[Meyer Jordan]

In particular, have you tested the source (well) water?

 

[sissy]

I am guessing here that koi are bigger than the comets .Usually bigger fish die first .Testing water is important .Have you taken the dead fish out right away .