Ugh....Messing with the factory siliconed bits on a "cheap" tank like a 150 or a 180 would bug me. Forget about it on a nasty custom tank like what you have.
Tough to tell from the picture, but is it the horizontal piece of glass you're looking to remove? Is it siliconed solid flat against another piece in the same direction/orientation?... sort of sandwiched? If so, it's gonna be a bear to get them apart without breaking the glass.
If it were me, I'd cut the background. I've seen a bunch of folks do it on other forums in build threads. The tricky part is getting a straight neat line. They say that one of those portable spinning lasers is the way to go for layout of the cut as it's tough to trace a straight line on a contoured surface.
Are you sure there's no other way to fit it in?