I had a 60 gallon tank do that to me and I just cleaned both edges really well, applied silicone and then applied pressure and let it sit for a few days and problem solved. I did also apply a nice bead of silicone to the inside corner of the tank as well, but after applying pressure to the edges to hold them together, this way the silicone on the inside holds the two pieces together from a different angle. Hope this helps.
Also, I know that HD sells small pieces of glass if you wanted to buy a piece and use that for your new seam, but the silicone should work just fine.