Tank Drilled...now it's leaking...Help!

[Michael]

I had my 72 gallon tank drilled on Saturday.
The store did a really good job....however, close to the drilled area...just on the other side of the overflow...it is leaking.
-I filled it up overnight and left it in the garage on a piece of plywood. In the morning, There was water on the side of the tank where it was drilled.
I don't know if it because the tank was old (1/2" thick glass) or what...but I need to fix it.

Any silicones I should stay away from when I reseal the bottom? Should I remove the old silicone first, or just silicone over it?
--note: the leak must be very small since the water level of the tank looked like it was the same height as when I filled it the night before. (but water on the floor).

Thanks,

Michael