I am in need of some advice and hope you could help.

I am having a fish die-off. They are goldfish and the pond is about 6 years
old. I have never had anything like this happen before. There aren't any
appearant signs of infection on the fish. The only symptoms that I see are
listlessness at the end of their life. They lose their appetite.

There haven't been any plant or fish additions to the pond. The only
connection that I have seen is this-toads come to mate in my pond as in
previous years. This year, I found a dead toad floating in my pond, which I
took out. That was about 3-4 weeks ago. Later, all of the tadpoles died
off. There currently are more and they seem to be doing all right. In the
past week, I have had 1-3 fish die every day. Is it possible that the toad
carried a virus that infected the fish?

I have done a water change this past weekend of about 30%. I medicated it
twice with a combination of formalin and malachite green. The pond is about
3,000 gallons and has an external filtering system.

I am at a loss. None of the diseases that I have seen described match this.

I am hoping someone will have some suggestions.

Thanks,

Ray