The wife bought a small goldfish at the weekend and put it into our
pond, well, to say pond its more like a large water feature really which
we have to attract wildlife.

I know we have newts in the garden and we have plenty of birds too, is
it likely that blackbirds or newts would eat a small goldfish. Cats
very rarely venture into our garden as its a rather enclosed garden and
they would struggle to escape quickly if disturbed but that could be
another possibility. There's a small pump running in the pond also but
its that small that fish couldn't get caught in it anyway.

just seems to be a mystery as to where it has gone but my theory would
be birds

Any ideas anyone?