Tank repair - Advice?

[AndyL]

Yeah, I could probably replace it and be done with it... But I kinda like these tanks (the stupid thick glass is awesome - except when lifting them )

Any thoughts on how to properly fix this - it's the bottom pane, it's at least 3/8" thick (50g 44x18x15) Sides sit on the bottom if it makes a difference.

My repair thoughts are along the lines of taking a piece of glass, rectangular in shape - 4" longer and 4" wider than the cracked area. Silicone it down, and around the edges. But I'm somewhat concerned about how the old/new silicone will react against eachother in this application - do I need to cut/clean the old off?