Sick Goldfish

Dear Ponders;
Yesterday my large (7") blonde goldfish did not come around to feed when I
tossed some floating food pellets in. I went looking for it, moved the
floating plants around and found it all alone near the surface in the
shallows.
It was not socializing with the other goldfish, shubunkins and small koi. It
did not look very healthy.
-bloated,
-scales were sticking out like a pinecone at the belly undersection and some
looked like they had fallen off.
-it is a blonde and it looked like it was bleeding or hemeroging on the
underside
-lethargic ( allowing me to net it)
-not eating

I looked it up on the Koivet.com Doc Johnson.
The symptoms sound like "Dropsey" and the prognosis sound grim!!! :(
My concern is for this fish but also for the health of my other fish.
Has anyone had experience with this? Should I remove? Isolate? Put it out of
misery?
Thanks,
Martha

PS. I have not tested the water but we did have a lot of rain in SW Ontario
this week and there is a bloom of algea going on in the pond 2000gal.