fish euthanasia

Alright, I've pretty much given up hope on one of my pond goldfish on ever
getting better, so I moved him into an indoor tank (after acclimating him to
the temp change) and waited a week for what I assume to be a severe slim
bladder disorder to heal. He was laying either on the top or bottom of the
pond, moving only when prodded and gasping for breath while the rest of his
pond mates seemed fine. I figured it would be easier to treat him inside,
in my hospital tank, but so far no luck. All my past experience for what I
assume to be dropsy (bulging scales, inability to swim rightside up or
maintain balance) the fish has either healed under treatment within a week
or died. This fish just won't do either though...

I've also fed him a thawed out pea just in case as well.

I feel that his options are pretty much gone and I'm not really interested
in seeing how long he can prolong his suffering, so my main question is the
best way to go about "ending" it for him. I realize how stupid this might
sound, but I rather liked this fish and have had him in my pond for three
years so I want to do this as quickly as I can. Any opinions? Or have
there been personally experienced cases where your own fish survived over a
week in this conditition to make a recovery?

I'll cross-post since this is a pond fish too, thanks in advance!

--Ryan