Goldfish staying and bottom of tank and not feeding

Hello everyone,
My daughter got a goldfish yesterday.It is sitting very syill at the bottom
of the tank and not feeding. I know that according to the FAQ that you
should ideally post before buying a fish but the deed is already done, and I
just want some advice on how to help the fish.
I have read on the net various reasons for this behaviour, from the type of
tap water in the area, to the water being too cold to poison.The first lot
of water I put in wasn't allowed to stand (I didn't know at the time) so I'm
going to change 50% today.If anybody could give me some advice I would be
very greatful.