Cut out all the silicone.  Make sure it is very clean.  I use a razor blade and then some alcohol after to make sure I have a very clean surface.  If possible you will want to put a small amount of silicon between the two panes of glass.  After you will need to make a uniform bead along the seams of the tank overlapping onto both panes of glass (use an existing aquarium for guidance).  I use my finger to push the bead into a nice looking seam.  Don't go overboard.  Less is really more in this situation.  I would rather go back and add a little extra silicone as I try to smooth the bead with my finger instead of having extra silicone spill out as I make the seam.  Look for air bubbles/gaps.  If done correctly you will not have any.

 

Hope that makes sense.  Good luck. 

I have a 24 gallon Nano cube that has a slow leak on the bottom back right side.  I am not the greatest at fixing leaking tanks.  I tried to reseal the bottom of the tank and it doesn't seem to work.  The weird thing was I sealed it up and put water in it about 1/5 the way up and sat it outside for 3 days.  There wasn't a leak in it.  When I filled it up and put rocks and water in it, the tank was fine.  Two days later it started leaking again.  Any help would greatly needed.