If it were my rimless build...
• I would use 3/8" glass (Starphire low iron if it is in the budget) with beveled (annealed) and polished edges.
• I would put the bottom in the center and silicone the side panes to the edges of the bottom pane. Not setting on top of the bottom pane. On a trimless tank, this will look much better.
• I would use Momentive RTV-108 (black) or RTV-103 (clear). This can be obtained at Grainger's, but is also readily available on Amazon and other on-line places. I would get the 10.1 ounce tube that fits in a caulking gun. To save the remainder (if there is any) I would recommend getting a DAP plastic caulking tube cap. I have tried everything I could think of and many things I read, but for me, the simple cap works the best for saving unused sealant for later use.
• Tape off your joints with painter's tape. Remove the tape immediately after applying and smoothing out the silicone. This will give you a straight and sharp edge to your siliconed joints. It will look much nicer, prevent a lot of mess and reduces time for clean-up.
• As mentioned above, don't squeeze all of the sealant out of the joint.
• After assembling the tank, use painter's tape to lightly hold the structure together and allow it to dry. I also always cover my work surface with wax paper, so the silicone will not stick to to the surface where you assemble the tank.
Hope that helps! Good luck and have fun with your project!