This has not been a good pond year for me. After five years of having no
problems at all, we had something (probably a raccoon) get in the pond and
clean out half the fish, including my oldest fish (about 10 years) that I'd
had since he was a little feeder fish living in a 3-gallon tank in my dorm
room at college. It happened because one night our waterfall got blocked by
leaves and drained about half the pond water before we found it, and some
predator took the opportunity to just wade into the shallow pond.

So, that was the beginning of the summer. Now this: last week my husband and
I started to notice one of our big old comets changing from orange to white.
At first we thought it was just a normal color change, but then we noticed
that her dorsal fin is also ragged. We netted her (a difficult ordeal) and
looked at her up close, and it appears that she's missing scales and the
white skin underneath is covered with little red dots. It reminded me of
ammonia burns (unfortunately I have experience with that due to a mishap with
some aquarium fish) but I tested the water and there is no detectable
ammonia. We decided to keep an eye on her to see if it got worse or better.

Today we were out feeding the fish and I noticed that other fish are showing
the same symptoms. Many of our fish are white or calico colored, so it
probably went unnoticed for a while. But now I can see that perhaps a third
of the fish in the pond have missing scales, irritated skin, and in some
cases also ragged dorsal fins. Most of the fish still appear fine. Those
fish that do have the symptoms aren't showing any change in their behavior --
their appetite is still fine and I haven't seen any flashing.

We haven't added any new fish or even plants to the pond in years, so there
isn't an obvious avenue for disease to have entered the pond. My hunch is
water quality problems, but we've never had to worry about the pond water
quality before so I don't really know what I should be testing. What should
I test for besides ammonia? pH, maybe?

Thanks,
Heather