fish dying in pond

First, I've always had pretty good luck with my pond.

I may have pushed it last weekend.

My pond has a mixture of some koi and some garden variety comet
goldfish.

Last Sunday I picked up some new koi as well as about 10 small "feeder"
type goldfish.

Introduced them all. All looked well... This morning I discovered 6
dead fish, both new additions and some older ones. All of these dead
fish were goldfish, not koi. My surviving fish look a bit lethargic
too.

For note, we did receive torrential rains this week and something may
have washed into the pond.

I'm sure, I've probably introduced some disease from the cheap comets.
Question is: What do I do now. Let it run its course, add drugs (have
lots of plants I don't want to kill).

Sorry, but I haven't had a kill off before.

Kevin