HI if you can find out exactly where the leek is it would make things a bit easer. as you should fill the hole some tanks have a gap between where the bottom glass meets the front and sides and back you maybe able to cut some of it out where the leek is. This would help to provent future leaks. clean your glass with only a water and vinigar solution. seems it is the bottom make the beed a bit thicker both on the bottom glass and on the walls you should redo all the seems at the same time that way you wont get any spots where the silicone will not bind to the old silicone. Once the silicone is cured fill with water to check for leaks leave for a couple of days empty and wipe the silicone I have noticed that some times I will get a slimy coating on it and aparently it is not good for your tank. Use aquarium safe silicone I get mone from RONA they seem to be the only ones around here that carry the large tubes. you can get it at LFS but they charge a tune for it, can you say rip off. Olso get the tool for smoothing the silicone out from rona or where ever you fine it where you are and use tape on the side corners make for a nice looking beed do the corners first, smooth them out remove tape and do the bottom. I have done many tanks myself I am in the process of fixing a 30g sump someone siliconed in a piece of glass over the broken area. it leaked. Now I am taking the bottom sheet out and the piece that they siliconed over the bottom. It has been a teedious and slow process, I still have to cut the new glass to fit.

Bill