I would also use a piece of glass that is much bigger that the hole, and at least the same thickness as the bottom. This should go inside the tank.
What I would do is scrape the tank surface with a razor, very very well, clean both the tank and the glass with rubbing alcohol, and then use a generous amount of silicone on the inside and outside. i would put down a thin layer, squish the glass onto it, and the do the seams around the outside of the patch(in the tank) and then the seams on the outside of the tank from the bottom.