New fish dying off - Help diagnose

I built a new pond in May and added four gold fishies soon after. They were
all doing quite well, until the local cat snagged one (Scarecrow stopped
working).

So about a week ago I added four new gold fishies (Lionheads). These are the
same type as the original (Walmart specials), but they were considerably
smaller than the existing fish, who eat like piggies. The new fish were also
a different colour, two were almost all silver with gold spots and one was
silver with speckles, no gold.

I let the new fish adjust to the water temp before I added them to the pond,
but did not adjust the PH.

The first new fish died within two days, laying on the bottom and not
eating. It ignored the other fish or they ignored him? The second new fish
(speckled) was active for a while, but did not eat and did not associate
with the other fish. Found him wedged under a rock on the bottom of the pond
this morning. It had a whitish coating on its body and eyes. The third fish
has always hung out with the other fish and eats like a horse.

The pond is about 300+ gallons, with water lillies, water hyacynth, small
fountain and waterfall. I have an external filter with UV light, so there is
no floating algea just surface algea. The PH runs a little high, but not too
bad.

Any ideas what might have caused this? Will it spread to the other fish?
Could it be an air bladder infection?