You could always try a product called Lexel. It is commonly used when people have an acrylic base on their shower or in their kitchen/bathroom, since most silicone's have a harder time keeping intact with it.. It's really a great product..I don't really know if having all glass and then one acrylic glass would really make a difference in your tank, but you could always try Lexel because it works great with acrylic. Most hardware stores sell Lexel. Here is their homepage and you type in your zip code and it will give you a store locater..
www.sashco.com/hi/lexel.html
I know quite a bit about caulking's cause I work at Lowe's
Wish the best of luck to your new project