Silcone repair

Hi
We have an old double concrete laundry sink that we have kept fantails in.
They have spawned last spring & have given nearly all the young to friends.
We've kept a few of the young separate from the parents, but their side of
the 'pond' is obviously leaking water, so we need to find the source of the
leak & repair it.
If we use silicone to repair the leak , how long would it take to be able to
put the fish back into the water?
I think they would all be able to be safe in one side of the 'pond' but
would be a bit crowded for a while, would this be OK?
I note that some ponds with Koi always seem to be overcrowded.
Thanks in advance for your help.
Pat