We have a half a dozen goldfish in an outside pond. They mainly appear
happy.

One is a pearl-scale goldfish. We didn't know this when we bought it and
didn't even know such a thing existed. Instead we worried that there was
something wrong as it turned into spherical fish with small fins.

Though it have never managed swimming very well and it has been
apparently unwell a few times before (months ago) it has usually seemed OK.

The past few days, though, has seen it go into decline. I added salt to
the water and turned on the fountain to get in more oxygen as these have
seemed to help before.

Sadly, though, it now spends most of its time upside down under the
lilly leaves. If I disturb it with a spray of water, it manages to swim
away quite well - actually quite vigorously. However it soon returns to
floating upside down not moving much at all, just breathing.

Recently the other fish have taken to following it around the pond and
appearing to bite it. I've understood that goldfish do that when one is
the goldfish equivalent of 'on-heat'.

What should I do?

My wife thinks that I should kill it to get it out of its misery, but
I'd like to find a way to get it back to its previously healthy state.
It doesn't look (apart from its behaviour) as if anything is wrong with
it - it hasn't rotting fins or anything like that.

--
For, methinks, the understanding is not much unlike a closet wholly
shut from light, with only some little openings left, to let in external
visible resemblances, or Ideas of things without. - John Locke, An Essay
Concerning Human Understanding (1690)
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