The poor thing now looks like a Puffer Fish.
I thought it was pregnant but it’s obviously not.
It’s so fat it’s scales are sticking out?
Presuming it has an illness. Can I save it?

Squeeze it

It’s got dropsy which is a form of fluid retention. Will probably die but a small chance of saving it. Buy some pond salt (not cooking salt) and mix up a strong salt solution in a separate tank or bucket. Put the fish in here for about half an hour and then return to the pond. Repeat if required the following few days.

Pond salt is the best forM of treatment for a lot of fish ills. It’s worth mixing up a strong solution and adding to the pond for general good health.. Always make sure it’s properly dissolved or the raw salt will burn the fish.

Thanks for that MTB-idle.
Will give it a go.
All it has to do is survive Christmas till the shops open again.

Dropsy is organ failure. It’s unlikely to recover. Lost a few fish this way. Take a water sample to pets at home in a clean Tupperware. I guess the water isn’t clean enough.

Go easy on the salt treatments too, they’re most likely to just stress the fish out. Try searching on the Practical Fishkeeping website, or fishforum.net for the right dose. Use pond water, not tap water because of the chlorine.