Please Help my fish are dying slowly

Hi, I am desperate I have a 50 gal preformed pond. My fish have been
fine health just fanncy goldfish up to 6 inches in length. Last week I
had a 5 inch one floating in the AM no trauma to him eyes clear no
gills and fins perfect. Yesterday again this morig again. I had the
water tested Ph a little high but we have had some rain everything else
fine. I added some plants a few weeks ago from a friends health pond.
NO they fish are near the type slugish kinda of gulping for air. These
fish are over 1 year old. I live in NY any sugestions, I was going to
get them out but not sure where trying to get a aquarium. If fish are
sick don't they look sick? Please PLease help...I don't even know where
I am going to get water from to put them in.

Carol