I have used my roof runoff successfully for a prior pond in my
backyard in Washington, DC. Both koi and goldfish survived for many
years. I had no additional aeration and relied upon fresh rainwater
to both flush the pond and provide oxygen. My roof at that time was
terne - tin-covered steel.

I have since put on a new roof, sheet copper with lead solder, and
have rebuilt my pond to be even larger. I'm now finding that most
fish die within 3-6 days after adding them to the pond, although some
have survived for almost 2 weeks. I am pretty certain that this is
because of the copper and lead leaching into the water (but I would
love to be proved wrong and find some other problem I can solve). I
hope that as my roof oxidizes and forms a patina the leaching will
decline and be less harmful to my fish. Does anyone know if this is
true? I am also looking for some kind of filter or treatment that can
remove the metal. My collection system includes a small culvert where
I could install a filter mechanism. Does anyone know of any kind of
mechanism/filter that can remove copper and/or lead? Is there any
kind of catalyst that can help?