We inherited a pond when we bought our house 2 years ago. There were
already 4 goldfish in it but we added two koi, one of which we never
saw after a few weeks.

Today, my gf was cleaning a bit of weed out when she netted a small,
green fish that had a silvery gold underside. One of the existing
goldfish had been looking a bit chubby recently(!) so is it possible
that it has spawned?

So, a few questions....
Are young goldfish green?

Yes, green to black.

Is it likely that only one would have survived or should we expect
more? (i.e. how many fish are expected from 'a spawn')

I bet more, especially if you have great hiding places.

Is there anything in particular we need to do to assist it?

Nope, leave it be.

When should we expect it to change colour? Xeno

They start to change within months to sometimes years, and sometimes never.
~ jan

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