Might want to get some RTV-108 Silicone. It is a great glass silicone, and is black.
My only hesitation would be that is may or may not bond plastic to glass. I think you would be happier with a black silicone, though.
Another thought would be to have the company send you a tube of silicone - it is their fault anyways.
Very nice tank David - I'm eagerly awaiting you getting this up and running. It will be very stunning, I'm sure of that!