I have an established pond - 2500 gallons, great filtration, 4 years old
- with about a dozen koi and a dozen or more goldfish. Koi are about 12
to 18" long, goldfish are about 2 to 3" long. Water lillies cover about
2/3 of the pond. Waterfall is a sheet of water about a foot across that
drops 2 feet, with smaller waterfall about 6" across makine 2 small
drops. Water is crystal clear.

Up until this past week I have had zero fatalities. But as of today I
have had 4 dead goldfish. They appear to have no parasites, though one
had it's fins chewed up (but that could have happened after he died).
But the koi all seem as happy as ever.

What should I do to start narrowing down the problem? Is it an oxygen
problem, or an imbalance problem, or??? Too many water lillies? I never
test the water - what should I test for?

Gary