sophie wrote:

the thing that was worrying me is that I think the gravel in the
little tank must be housing a lot of the bacteria, and if I go for the
(admittedly bonkers) sand idea, I won't be transferring the gravel. In
fact I don't think I'll be transferring the gravel anyway, as the
goldfish keep sucking it up and spitting it out, and I'm a bit worried
that one day a piece will get stuck

Take some gravel, put it in a little bag/pouch make of cheese cloth or a
nylon stocking. That way the buggies can hitch a ride to the new tank,
but you don't need to worry about cleaning up the gravel later...

My $0.02.

-Donald
--
"A positive attitude may not solve all your problems, but it will annoy
enough people to make it worth the effort." -Herm Albright