Random thoughts, in addition to what everyone else
is posting.
Are these the only fish in the pond, the ones who keep
dying? Or if you have other fish are they looking peaked too?
Or
Has anyone been doing any spraying in your area, fruit trees,
etc?
Anything new on your lawn that could get into the pond?
Also
If you have other options I'd try another fish store, their
stock might be healthier.
Or if I had the option I'd adopt some overstock from another
ponder.
Has your city or county water supplier started adding
chloramines to your system?

good luck!
kathy