leak in concrete pond - a little help?

It's in the garden of the local pub. There's been a leak for
months/years, but only occasionally. We refill the pond and forget
about it. Now it's almost emptying every day. I put talcum powder on
the surface to see if it would be pulled to one side, illustrating the
whereabouts of the leak. several hours later, the pond had gone down
loads and the talc had stayed in exactly the same place. This leads me
to assume that the leak is smack dab on the bottom. Could any kind
souls advise me on a) how I can find the leak, b) how I can fix it and
c) what to keep the handful of fish in while I'm mending the pond?

BB
MFW Britpack www.extreme-positioning.co.uk
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